Water Chemistry for New Aquarists (Taken From MOA's Keeping Fish)

The Dynamic Fluid



            “Water is a universal solvent”—it is a phrase often stated in many high school science classes all over the country (US), but it is not entirely accurate. For example, water cannot eat its way through glass nor will it absorb oil into its solution the same way in which it absorbs most salts. Nonetheless, despite not being a truly “universal” solvent, water is certainly a dynamic fluid with many properties that separate it from other fluids. The distinct properties of water are the foundations of life and all existence known to man. Specifically, water is the chemical that aquarium husbandry owes its existence to.

            Because the properties of water can affect aquatic animals to a much greater degree than their terrestrial counterparts, Keeping Fish has an entire chapter devoted to the subject of water chemistry. This chapter is not exhaustive, by any means, but, hopefully, it will provide aquarists with relatively solid explanations of some of the basic factors affecting their fish. The goal of this chapter is to inform aquarists of basic water chemistry and help them avoid potential problems that may arise due to the various components of water chemistry.



Water as an Active Solvent


            Water is active, almost ridiculously active. Consider this, in 18.016 grams of pure water there are approximately 6.02214×1023 molecules (Avogadro’s number). Each one of these molecules is hurtling through the medium and bouncing into each other (and their container) with nearly elastic collisions (meaning that they do not lose energy). If one could harness all the energy of these molecules, then there would be more than enough energy to light the entire city of New York. This same molecular composition is what our aquatic pets live in; aquarists keep animals alive in one of the most dynamic (changing) substances ever discovered.

            In addition to being active, water is also somewhat unusual in that it has both distinct molecules and a polar construction (polar means that each end of the molecule has an opposite charge, either positive or negative). With many other polar substances, the strength of the respective charges pulls the substance into a single, solid mass at room temperature. An example of such a substance is table salt (NaCl). Each grain of salt has no distinct molecules, just a series of interconnected ionic bonds. These substances, as they have no distinct molecules, tend to be brittle and abrasive. Water, on the other hand, has internal attraction (called cohesion) but is not so strongly associated that it forms a solid (at room temperature, at least). This is what makes water the perfect liquid for sustaining life: it is active, semi-polar, and resists the solid state.

            Because of water’s unique nature, it often behaves as an absorber of other substances in that it takes them into suspension within the liquid. Most liquids will only absorb very specific substances. As an illustration, oil only absorbs oil-like (non-polar) substances. Water, though, can absorb a broad spectrum of substances because it is not strongly polar, only weakly polar (unlike salt). Thus, water can absorb non-polar substances and polar substance because it has properties of both. This property of taking on other substances is called being a solvent. The substances that are absorbed are called solutes. In more common terms, many substances dissolve or diffuse into water.

Since water is such an active solvent, most natural water sources contain many other chemicals in addition to just water. Even a city water supply may contain at least ten other significant (easily measurable) substances besides H2O (water). The number of possible trace substances (those impractical to measure in most cases) could easily be two dozen or more. Even at that, no less, city tap water is devoid (or nearly devoid) of many substances normally found in naturally occurring water sources. The concentration and composition of all these other substances is what water chemistry is all about. Water, in itself, is fairly easy to account for, but its dynamic nature means that whenever you are dealing with water you are also going to be dealing with a myriad of other substances.

New aquarists (or even experienced ones) who assume that all water is the same often wind up with dead fish. Why? The answer is simple: fish are sensitive to the conditions of their aquatic environment in much the same way that we are sensitive to impurities in the air we breathe (Ever lived in a smog-laden city?). Of course, one can go too far in accounting for water chemistry and it is not something that needs to be constantly worried about, but a basic awareness has not hurt anyone yet. In short, knowing water chemistry is what gives aquarists an upper hand in correctly diagnosing problems and avoiding potential problems.


Reference for Water as an Active Solvent:

Ø  Silberberg, MS. Chemistry: the molecular nature of matter and change. Boston: McGraw-Hill; 2006. (fourth ed.).



Salt, Osmosis, and Gradient Flow


            Salt is a touchy subject in the aquarium world, particularly on FishChannel.com, because it represents a somewhat complex chemical process and a chemical hardly ever found in freshwater. Table salt (and Kosher salt) are composed mostly of sodium chloride, which is rare in most freshwater environments. In fact, it is also somewhat scarce in seawater in that many more salts are present than just sodium chloride (seawater contains many minerals and many salts combined to form a very complex solution). Despite the aberrant nature of salt (table salt, that is), which I will refer to hereafter as NaCl to avoid confusion with other salts, it is still a very popular aquarium additive. Why is NaCl so common? The answer is complex and involves osmosis and some other chemical properties.

            Osmosis is the “process by which solvent flows through a semipermeable membrane from a dilute to a concentrated solution” (Silberberg, G-12). Put another way, fluids tend to try to balance their concentrations if separated by gradient through which they can flow. As an example, imagine a cup of water. If you let a drop of food coloring fall into the cup, the food coloring will become dispersed thorough the liquid so that, eventually, the entire cup of water takes on the color of the food coloring. However, the color of the water in the cup will not be as dark as the original drop of food coloring due to a diminished concentration. The same sort of thing happens in the home aquarium: if you add something to the water (or remove something), then the water will try to rebalance its concentrations both in and outside of the fish.

            This is why NaCl can be significant: If you add NaCl to the water, then the fish’s cells must give up water in order to maintain osmotic balance. In other words, adding NaCl effectually dehydrates your fish. Subsequently, many FishChannel.com users feel that NaCl, in most cases, creates more stress than it is worth. However, concrete documentation on NaCl being absolutely good or absolutely bad is rather scarce. Therefore, FishChannel.com users tend to go back to Best Practices and state that since NaCl is both unnatural and possibly stress-inducing, that it should not be added to any aquarium unless special conditions warrant its use.



            In all fairness, however, NaCl has been observed to reduce the effects of nitrite poisoning, has been used successfully in the cure of ich disease, and can be useful as a mild tonic for high-turnover fish stores, etc.

            Here is what a few FishChannel.com users had to say regarding salt in freshwater aquariums:


math-only aquarium

“One problem with salt in freshwater is that its short-term effects are predictable while its long-term effects are not. In the short run, a little added salt can protect against nitrite and irritates the fish so that they produce a thicker slime coat. This can prevent them from catching most mild pathogens if they are in otherwise good health. It is kind [of] like you or I running a mild temperature when we get sick—it can kill some basic pathogens. Because of this benefit, many places that keep fish short-term use salt regularly. Also, salt is advisable if you are cycling a new tank [with fish].

“However, prolonged exposure results in stress due to osmotic imbalances. It is hard to predict when this occurs or even if it occurs in some species, but those that do suffer (and most freshwater species do eventually) can lose kidney functioning, [have] organ problems, and even [suffer] neural damage (theoretically).

“As it is hard to tell when these internal changes occur, many aquarists avoid salt altogether. Instead, they simply keep the tank clean by doing frequent water changes and minimize stress by not overcrowding the tank. Statistically, cleaning frequency and stress reduction are the best predictors of fish lifespan. Salt, on the other hand, often trends along with high stress, bad fish combinations, and a lack of attentiveness. In other words, people with salt tend to get lazy and stop caring for their fish like they should; salt enables instead of empowers.”


David Lass

“…In my wholesale fish room I do keep a plastic container filled with Kosher salt, although I [do not] use it very often. Many local fish stores and other sources of wisdom about [keeping fish] often recommend salt as a general "tonic" for fish. The usual dosage is between a teaspoon per five gallons to a tablespoon per eight gallons, or thereabouts. [I am] not a big fan of such use. At those dosages, I [do not] really think there is much therapeutic benefit—I think it serves more to assuage the hobbyist's need to ‘do something’ for their fish. As a cure for things such as ich, there are a number of remedies out there that will do a better job, faster, and without messing around with raising the temperature of the tank.

 “The fish lore also has it that salt is good for use with mollies, other livebearers, and goldfish. This I also think is wrong. The vast majority of livebearers, including mollies of all types and colors, and sailfins, come from the Far East. They have been raised in water that is moderately hard, and of neutral pH. These fish are very far removed from the wild original mollies that came from brackish water. I typically will have in my wholesale fish room a few types of sailfin and lyretail mollies, balloon bellies, blacks, reds, dalmatians, and a few others. All of them do fine without salt. The same with goldfish. I think that what is going on here is a confusion between fish that need salt and the fact that they need alkalinity. Salt is just one part of alkalinity, and my experience has been that raising alkalinity by keeping chunks of ‘Utah Ice,’ which is a trade name for raw gypsum, is what is required for goldfish, and some livebearers. [Editor’s Note: {I, personally, have always seen better results with adding NaCl to aquariums for mollies and other livebearers if given the option between unaltered freshwater and adding salt. No less, raising the alkalinity has proven better than both.}]

“Adding salt to water for brackish water fish is an entirely different thing, and I agree with what has been pointed out on [FishChannel.com] that for this purpose it is best to use [a] marine salt mix. The other thing that salt has been mentioned for is to help fish with osmotic problems, especially after having been shipped in, or living in, water where the nitrite and nitrate are very high. Salt does help stressed fish with regulating their [osmotic balance], but I have found that using furazone green is a better, and faster, way to address the problem.

“So, that [is] my take on things with salt. It probably [cannot] do any harm, but in the dosages recommended it probably is [not] doing much good either. If you do decide to use salt, plain Kosher salt or aquarium salt are much better to use that any table salt you may have around the kitchen.”



“The reason most people choose salt and [temperature adjustment] for cases like ich and a few other things is really quite simple. They [do not] want to use indiscriminate [and potentially harmful chemical remedies]. The temperature/salt method works and it works fast if you catch the problem before it gets out of control.

“Most of us are hobbyists, not wholesalers. We do not have huge revolving stock that changes daily. We pay a great deal of attention to our limited amount of individual fish on a daily basis. If I see a few [salt-like] grains on a fish, it only takes a minute for me to turn the temp up and add a little salt [to a] tank. It takes me a half hour to go to the store and buy medicine. The use of salt just to use salt is a bad practice created by OLD, [outdated] aquarium literature.

“The absolute best practice to prevent disease and resolve minor fish ailments is the same as it always has been: HIGH QUALITY WATER/LIVING CONDITIONS for your fish.

“Some fish loses happen and we can do nothing about it. The vast majority is just human negligence or bad husbandry.”



“The interesting thing about this is that I was looking up studies on salt tolerances and the uses of salt in combating different diseases. One of them claims that marine salt, in low concentration, is actually beneficial. I reviewed the study, and while I question why they felt the need to test it in the first place, their methods are sound, and it really does appear that marine salt could be beneficial, provided you keep it low. The only thing I could figure was that in addition to increasing the ions in the water, for gill transfer, you are also increasing the calcium and potassium in the water, which have definite benefits for your fish, especially if they are lacking in your tap water or the food you are feeding.”


            The NaCl debate will probably continue for years, but it is generally accepted that adding anything overtly unnatural and unnecessary to the aquarium is not in keeping with Best Practices. As such is the case, it is the stance of Keeping Fish that NaCl should not be used unless a good, specific reason can be given for its use. Furthermore, use as a general tonic or stress-reducer is unwise (as NaCl can actually create stress instead of relieve it).


References for Salt, Osmosis, and Gradient Flow:





Ø  Silberberg, MS. Chemistry: the molecular nature of matter and change. Boston: McGraw-Hill; 2006. (fourth ed.).



Water Hardness, pH, and Alkalinity


            As mentioned, water can contain a plethora of different chemicals with varying properties. This creates a problem for anyone who deals with aquatic systems on a regular basis: How do you describe something that can be composed of one out of millions of chemical constitutions? While there is no one schema that will describe any body of water, scientists did come up with three measures that can give us a fairly precise means of describing water and predict how it will change: water hardness, pH, and alkalinity.

            As the intricacies of these three measures are somewhat complex, Keeping Fish presents in-depth discussions via a few posts by FishChannel.com users and follows such with a synopsis of the three measures that simplifies their respective details. Also, the references to this section of the book include additional data that can be useful if greater understanding is desired. (NOTE: Almost all posts were edited to unify terms.)


David Lass

“…Three things basically matter for your fish when it comes to the water they live in: 1) pH: how acidic or basic the water is; 2) hardness: amount of minerals in the water; 3) alkalinity: [basically the] buffering capacity [of the water].

“Aquarium water usually is either soft and acidic (low pH and low mineral content) or hard and basic (high pH and high mineral content). The other factor, buffering capacity [or alkalinity], simply means the tendency of the water to resist changes in pH.

“For example, my water comes out of the tap at a high pH (8.5), but has low hardness and very little buffering capacity. I can do a 30% water change on a tank that has a pH of 6.8 and within a short period of time it is back at that pH. Hard/basic water is what many folks, especially in the Midwest and West have (which is why African cichlids work so well for them, since [African cichlids] like…hard/basic water).

“The most dangerous situation is what I have, since with little or no buffering capacity the pH of the water can change dramatically. It can drop from a safe level in the 6's to a lethal 4.5 or lower in a matter of days. That is why I keep pieces of marine base rock and gypsum (Utah Ice) in my tanks, and aragonite in filters in my wholesale fish room…”



pH: Concentration of hydrogen ions. determines if water is acid or base. Put another way, it is the amount of free acid in the water.

Kh: Referred to as buffering capacity and determines the ability of the water to absorb hydrogen ions. Measured by the amount of carbonates in the water.

Gh: Referred to as [general] hardness. It is the total combination of dissolved solids in the water and includes carbonates.

“Kh is the key. Kh binds with acid or releases it depending on the base material the carbonate is bound with. Sodium bicarbonate has a very high natural pH and readily binds with any hydrogen ions (acid). [Phosphor-based buffers,] however, bind poorly with hydrogen ions and tend to have a low pH. Since carbonate can be combined with many other elements, there are a large number of different buffering agents that can be created to suit our needs.

“Because of the diverse nature of buffering compounds, Kh, by itself, does not fully determine the pH of the water. It really is only a measure of the capacity that water has to resist changing of the pH. If the buffering compound naturally seeks a high pH, like sodium bicarbonate, then attempting to force the pH lower by adding acid will tend to result in problems. In order to effect the pH change, all the buffering capacity must be used up and any additional acid will cause a large change in pH. The opposite is also true for compounds that have a naturally low pH. Large amounts of acid must be removed from the water to cause a rise in pH, and once the buffering compound is out of acid then the pH will rise very quickly.

“To effect a change in pH without causing problems, you need to determine the natural pH of the buffer and either add buffer with the opposite pH or reduce the buffering capacity by adding water lacking in Kh (also known as soft or distilled water).



Low Kh, Low pH: Add sodium bicarbonate (baking soda) to increase both Kh and pH.

High Kh, low pH: Add sodium bicarbonate and (maybe) distilled water to raise Kh and pH.

Low Kh, High pH: Add [phosphor-based buffer] to reduce pH and raise Kh.

High Kh, High pH: Add distilled water and [a phosphor-based carbonate] to lower pH.


bto83 [sic]

“Sorry I didn't answer this sooner, but it's a long reply (or should be) and I just really didn't have time to write it until now. I'm going to "dumb down" the explanation and try to avoid any confusing jargon. If something seems confusing, try plowing through anyway and get to the end (where other definitions are included) and then hopefully it will make sense if you re-read it.

“First off, I figure I'd better clarify the basics and terminology.

“Water is self-ionizing: in other words, you could say it "dissolves itself." For basic purposes, we'll just say it dissociates into H+ and OH-. This reaction goes back and forth easily and routinely through random motion of molecules, with the equilibrium it settles upon being referred to as it's pH.

“pH: the measure of H+ ions in the water, based on a logarithmic scale. Implicitly, this can also be defined as the relative difference between H+ and OH- ions. At a pH of 7, these two ions are expected to be in fully equal proportions. At pH's lower than 7, the number of H+ ions is greater than the number of OH- ions which is called an acidic condition. The opposite is true at pH's above 7, which is called a basic condition.

Alkalinity: refers to a solution's ability to neutralize H+ ions (strictly speaking, this term is only in relation to carbonate equivalence points). Read the link for a more exact definition if you like. This is related to buffering capacity, which refers to a solution's resistance to a change in pH -- its ability to keep the H+ and OH- ions at a particular balance. The important thing to note is that alkaline does not mean a pH above 7 or mean non-acidic. It's frequently used that way, but that's incorrect and more importantly it's quite confusing. As stated above, a pH above 7 is properly referred to as basic.

“How buffering works: it requires other ions to be dissolved in the solution, and these other ions will gain or release either H+ or OH- ions. The most common by far is bicarbonate (HCO3-). It can:
1) release an H+ ion to become carbonate (CO3--), which thusly increases the number of H+ ions and lowers pH (lowering the pH is meant to resist an upward swing in pH caused by something else) or
2) it can absorb an H+ ion to become carbonic acid (H2CO3) which thusly lowers the number of H+ ions and increases pH (meant to resist a downward swing in pH).

“Carbonic acid can do #1 while carbonate can do #2, but neither can do both. Thusly a solution of mostly bicarbonate is more stable than a solution composed largely of either of its other forms because they can only resist in one direction.

“Within aquaria, bicarbonate and carbonic acid are the predominant forms while carbonate will only exist in small or trace amounts. If something happens that should change the pH (for instance, an acid releasing H+ ions), this will cause a shift in the H2CO3/HCO3-/CO3-- percentages (they will gain/release H+ ions and change form) and this shift will help to negate the pH change as described above. The amount of pH change that can be resisted is directly tied to amount of carbonates in the system and what form they are in. Within aquaria, downward shifts in pH are usually a more common concern than upward shifts which means bicarbonate is the key buffer (because carbonic acid cannot absorb any additional H+ ions and carbonate is rare).

“The most stable pH's are where the lines look nearly horizontal in the graph. The steep lines and crossover points can be very unstable (sometimes even with hard water) because the carbonates undergo such radical shifts in their relative concentrations at these points.

“Carbon dioxide (CO2): Why does adding CO2 lower the pH? When CO2 is added to water, much of it undergoes the following reaction:

CO2 + H2O => H2CO3

“In other words, it forms carbonic acid. This reaction is pH-neutral - it does not change the pH. However, portions of the carbonic acid may immediately dissociate via

H2CO3 => H+ + HCO3-

“which quite clearly releases an H+ ion, lowering pH. The extent to which this happens depends on pH, as the chart illustrates. Above ~8.0 practically 100% of the carbonic acid will dissociate into bicarbonate and hydrogen ions, meaning all of it will work to lower pH. At 6.0 most of the carbonic acid will be content to stay as carbonic acid, and only about 30% of it will work to lower the pH. As pH descends, less CO2 will impact the pH.

“Other buffers: carbonates are the most common buffer, but there are plenty of others out there. Calcium carbonate (limestone) is one of the predominant ways carbonate enters water supplies, and calcium (Ca++) itself is a buffer. It combines with OH- ions to form Ca(OH)2, which decreases the number of free OH- ions in solution to lower pH. This reaction, like all other buffer reactions, depends on pH. It occurs more frequently as OH- concentrations grow (pH's above 7). Even though it mainly only proceeds with free OH- ions, the reaction is written as:

(Ca++) + 2(H2O) => Ca(OH)2 + 2(H+)

“This is the general effect it has, because absorbing free OH- ions causes H2O molecules to dissociate to form OH- (as replacements) and H+. This is the crux of why pH can be interpreted as the relative difference between OH- and H+ ions -- removing one will cause in increase in the concentration of the other.

“Copper does the same thing, forming Cu(OH)2. Copper is inert in this form, so if you ever decide to dose with copper as a medicine, it's best to look up dosage instructions somewhere that account for it's pH specificity. A safe amount of copper at 8.0 can be deadly at 6.0, while a safe amount at 6.0 may have no therapeutic effect at 8.0.

“[One commercial buffering agent] utilitizes phosphate (PO4---), which is even more complex than carbonate. It's forms are PO4---, HPO4--, H2PO4- and H3PO4, and encompasses an even wider range of pH's than carbonates do. One of it's horizontal lines happens to be around 6.5, which makes it a good buffer if that's your ideal pH. Depending on your source water, you may already have quite a bit of it. It can be a bad idea to use it in planted tanks though, since it will likely cause a huge algae bloom and wild pH swings (after the algae eat all your buffer).

“Adding buffers: should be done carefully, and with regard to the form in which the buffer is added. For instance, adding sodium bicarbonate (baking soda) to water with a pH of 8.0 will show no change in pH almost regardless of how much you add because ~100% of the bicarbonate will stay as bicarbonate and thusly be pH-neutral. Adding baking soda to water with a pH of 6.0 will cause the pH to rise, because nearly 70% of the bicarbonate will absorb an H+ ion to become carbonic acid. Adding baking soda to water with a pH of 12.0 will lower the pH because it will give up an H+ ion to become carbonate.

“Adding carbonate (through, say, calcium carbonate from coral or shells) will raise the pH of any solution less than ~13 because it will absorb one or two H+ ions.

“If you're not careful, adding buffers can swing your pH very fast especially if you're at one of the cross-over points. Always make sure you know what ion you're adding and what it's going to do when it hits the water before you actually add it.

“Carbonate Hardness (KH): Is a means of measuring alkalinity, but in a somewhat complicated way. It's designed so that one ppm of carbonate is worth two ppm of bicarbonate, and carbonic acid is not accounted for at all. In other words, it basically measures your water's ability to absorb H+ ions through carbonates (their ability to resistant a drop in pH). If the pH of your water is about 8.2 - where ~100% of the carbonates are in bicarbonate form - then KH is also equal to the ability of carbonates to release H+ ions (their ability to resist an increase in pH). If your pH is some other value, you will have to calculate this value yourself - a pH lower than ~8.2 will have a greater ability to resist an increase in pH than KH states while a pH above ~8.2 will have a lower ability than KH states. It is not supposed to take into account other buffers at all, so if you add phosphate buffers it will not change KH.

“Acid Neutralizing Capacity: Similar to KH, but will take into account any ion that can absorb H+ (for instance, phosphate). In other words, this is your water's total ability to resist a drop in pH. I believe that some kits claiming to measure KH actually measure this value instead. You cannot accurately infer your water's ability to resist an increase in pH with this.

“Buffering Capacity: This would be your water's total ability to combat both an increase or decrease in pH from all buffering ions. There is no way to measure this with a single test.

“General Hardness (GH): The measure of "hard" ions in the water, such as calcium and magnesium. In general, this is not a very useful thing to measure but it can give you an idea of your water's ability to absorb OH- ions.

“Total Dissolved Solids (TDS): The total amount of "stuff" in the water. This includes everything, whether it's a buffer or not. A very high TDS can upset some fish and cause them to lose buoyancy as the density of the water becomes too high…”


            Given that hardness, pH, and alkalinity can be hard to understand, Keeping Fish includes the following selections as a general (though not precise) guide to water chemistry fundamentals:


Water Hardness


            Working Definition: The total measure of dissolved substances, especially multivalent cations like calcium and magnesium, found in a given body of water expressed as a concentration.

            Scale: While there are many different hardness scales, one, as per Ines Scheurman, is as follows:


Very Soft Water

Below 75ppm*

Soft Water

75 -150ppm

Medium Hard Water

150 - 220ppm

Hard Water

220 - 360ppm

Very Hard

Above 360ppm

*parts per million; same as milligrams per liter


            Significance: As hardness is essentially a measure that compares the concentration of substances in bodies of water, it primarily affects fish through osmosis. Subsequently, fish from soft water will release water from their cells if placed in harder water than what they are adjusted to. This means that fish from comparatively soft water can become dehydrated in hard water. Conversely, fish adjusted to hard water will absorb water into their cells to maintain osmotic balance if placed in soft water. In fact, so much water can enter the fish that its cells will literally burst open.

            Additionally, water hardness can also affect buoyancy and other chemical properties such as pH and alkalinity, though the effect is usually comparatively marginal. With regard to buoyancy, the difference between the adjusted hardness and the new hardness has to be very large to produce an appreciable effect—almost as significant as placing a saltwater fish in freshwater. On the other hand, pH and alkalinity are properties that derive their values directly from the constitution of the water and thus are more strongly associated to hardness.

            General Notes: Most of the fish commonly sold in the aquarium hobby are amazingly adaptable and can survive in a myriad of potential water hardness values, even if far from ideal. As a case in point, neon tetras, which originate in very soft South American river systems (less than 150ppm), have been adjusted to hard water that is more appropriate for keeping African cichlids from the interior lakes (more than 300ppm). No less, such a radical adjustment cannot be instantaneous.

            Instead, while most aquarium fish can be adjusted to most water hardness values, fish are more likely to survive if changes are very gradual ones. As has been said, “Fish can tolerate change, but those changes must be reasonable and no more than we would expect of ourselves.” As an example of this principle of reasonable expectations, imagine that water hardness is like oxygen concentration at high altitudes: the more drastic the difference between the altitude one is used to and the altitude that is newly encountered, the more severe the adverse reactions. Also, we would not expect someone who has spent all of her/his life on the seacoast to be a suitable candidate for an immediate expedition to the summit of Mt. Everest. Similarly, we cannot expect great feats from our pet fish.

            As a general rule, fish should never be expected to adjust to a water hardness more than one classification away from what they are accustomed to. That is, a fish from very soft water will not do well (and probably will not survive) if immediately placed in medium hard water. Conversely, a conversion from very soft to soft water will probably be successful if the transition is slow enough. Because of the importance of ensuring similar hardness levels, it is always prudent to check the water of the pet shop you plan on purchasing fish from before making a purchase. Try to find a shop that is within the classification of your own aquarium water so that the transition will be as smooth as possible. (NOTE: Such actions do not negate proper acclimation of the fish, which will be discussed later in Keeping Fish.)




            Working Definition: The pH number is a symbol for the negative, base ten logarithm of the concentration of hydrogen ions in atom grams per liter.

            Scale: Unlike some other measures, pH is relatively fixed and it is generally accepted that a pH of 7.0 is neutral, below 7.0 is acidic, and above 7.0 is basic.


pH Range



Less Than 4.0




Dangerous for Most Fish



Relatively Unstable



Relatively Stable



Neutral and Stable



Good for Many Fish



Tolerable for Some Fish





Greater Than 11.0




            Significance: This measure directly correlates to acid-base properties and thus affects fish much more directly than water hardness. Since the scale is logarithmic, a difference in one unit on the pH scale represents a tenfold difference in potency. Consequently, a pH of 7.0 is 10 times more acidic than a pH of 8.0 while the pH of 8.0 is 10 times more basic than the pH of 7.0. This condition is further compounded as the difference in units increases. For example, a difference in 2.5 units on the pH scale represents an approximately 316-fold increase or decrease in concentration (102.5 ≈ 316.227766). Such a difference would be like the contrast between sticking your hand in water and sticking it in battery acid!

(Next Column) 
General Notes: Since the pH scale represents such drastic differences in concentrations, it is absolutely imperative that fish not be required to adjust to a value too unlike the one they are accustomed to. Nonetheless, as with hardness, most fish can be adjusted to a wide range of pH values if the transition occurs over a sufficiently large period of time. With this in mind, it is often recommended that fish are never transferred between waters that have a pH difference greater than 0.2-0.5 units depending on the sensitivity of the species. For example, guppies tend to be very hardy and can handle most common pH values given time to adjust, but cardinal tetras, on the other hand, are much pickier and often do not adapt to large changes regardless of how carefully they are acclimated.

            Another important consideration is that the biological processes that occur in the typical freshwater aquarium tend to bring pH down (make the water more acidic). This is perfectly normal and usually does not merit special consideration. However, if the bioload is large (you are keeping many animals), then it will be prudent to add a buffering agent (increase the alkalinity) before the animals are in the tank. Also, some aquarists fret over a high pH and the truth is that most biological process bring the pH within suitable parameters after the tank has been running for a few weeks, but testing the water and being wary of sudden changes is still important.

            Moreover, as with water hardness, it is important to check the pH of a potential pet shop before buying fish from the establishment. Sadly, many aquarists, particularly new aquarists, often try to achieve the “perfect” water chemistry in their own tank—sometimes artificially altering the pH to fit some prescribed mandate (like the “perfect 7.0”)—and fail to realize that most pet shops do not take such measures unless the fish are inordinately sensitive. This being the case, the fish in the pet shops are adjusted to conditions that the aquarist is not providing and later die upon entering the new tank. The moral of the story is that altering your own water chemistry is often unnecessary unless it is very different from the pet shop or you are trying to breed fish (or undertake another specialized goal).




            Working Definition: The measure of dissolved substances that resist changes in pH; a special subclass of general hardness.

            Scales: There are many alkalinity scales that are commonly used and can be found using the references at the end of this section.

            Substances: Items that increase or alter alkalinity include the following:

·         Crushed Coral

·         Limestone

·         Baking Soda

·         Aragonite

·         Egg Shells

·         Gypsum

            Significance: Alkalinity prevents the pH from changing quickly and thus helps maintain constant aquarium dynamics. This means that an aquarium with buffers added to it will be less likely to stress the fish. Also, adding buffers saves the aquarist some mental anguish in that he or she does not have to worry about easily upsetting the chemical balances.

            General Notes: While adding a buffer is recommended by Keeping Fish, it should be noted that the particular buffer added is of special concern, as is the quantity used. If one is unsure of how much buffer to use, it is best to keep the dosage under one-fourth of a teaspoon per ten gallons (less than 0.033 ml/L). At this dosage, it is unlikely that the buffer will cause significant damage if it is of an inappropriate sort—or, better yet, do not use it at all if you are unsure about its effect. Also, gypsum is often the safest buffer for the average aquarist (though cross-referencing your buffer with your pH is still wise).


Concentration Reduction Formula


            Occasionally an aquarist has a very high water hardness that he or she wants to bring down for a special project such as breeding or keeping a delicate species. One way to soften water that is relatively feasible over the short term is to add distilled water to the hard tap water. The resulting solution will have a lower hardness rating and can be used as aquarium water.

            The sticky issue for many aquarists is just how much distilled water needs to be added to bring the hardness down to a specific desired level. Fortunately, there is a relatively exact formula for such an occasion and is as follows:

Distilled Water = (Gallon Size of Aquarium)*(1-(ppm desired)/(ppm of tap water or aquarium))


D = G*(1-desired/have)

            As an example of how to use this equation, suppose we have a 10-gallon aquarium with a hardness of 220ppm and we want a hardness of 130ppm. To figure out how many gallons of distilled water we need we would divide 130 by 220, subtract this quotient from one, and multiply the result by the tank size. In this case, the number of gallons of distilled water would be 4.09 gallons (Dw = 10×(1-130/220); Dw = 10×(1-0.590909…); Dw = 10×(0.409090…); Dw ≈ 4.09 gal.). Correspondingly, the amount of regular aquarium/tap water would be about 5.91 gallons (10 gal. – 4.09 gal. ≈ 5.91 gal.).

            This formula can be handy in that it works for any case in which a stable concentration is being reduced. Nonetheless, it does not account for cases where a concentration must be increased. Lastly, such a dramatic change as purported by this formula should never be done with live fish in the aquarium system. If live fish are present, the necessary amount of distilled water should be added over the course of several weeks and multiple water changes (with the amount of distilled water per replacement volume increasing to the desired proportion). Luckily, this equation also works for smaller containers like the buckets used to exchange aquarium water: just substitute the gallon capacity of the bucket for the gallon size of the aquarium in the formula.


Best Practices Synopsis


            The most important elements of fundamental water chemistry are as follows:

·        Regularly Test Your Own Water

·        Check the Water Chemistry of Local Pet Shops

·        If in Doubt, Don’t Mess With It

·        Remember that Fish Adjust Slowly

·        Remember that Fish can Adjust to Most Chemistries


References for Water Hardness, pH, and Alkalinity:






Ø  Bailey M, Sandford G. The ultimate aquarium: a definitive guide to identifying and keeping freshwater and marine fishes. New York: Smithmark; 1995.

Ø  Scheurman, I. The natural aquarium handbook. New York: Barron’s; 2000.

Ø  Silberberg, MS. Chemistry: the molecular nature of matter and change. Boston: McGraw-Hill; 2006. (fourth ed.).



Dissolved Gases


            One important group of substances that cannot be omitted from freshwater aquarium husbandry are gases. In fact, without them, there would be no chance of keeping fish alive as almost all fish require dissolved oxygen to survive. Alternatively, some gases are quite toxic and can kill fish in low concentrations. The toxicity of other gases depends on the balance of the constituent gases in an aquarium and their relative concentrations.

            Since gases can and do affect fish, Keeping Fish has dedicated this section to the subject. As with many other scientific sections in this book, this section is condensed and not entirely accurate. Instead, it gives the pertinent information by way of analogies and abbreviates most of the more intricate data.


Gas Sources and the Equilibrium Point


            Gases can originate in one of two places: within or without of the water. This is usually not very surprising to anyone, and makes a discussion of how gases operate in the typical aquarium a rather easy matter. Before much more is discussed, though, it is necessary to delineate the major gases found in the average freshwater aquarium and where they come from:





Oxygen (O2)

Atmosphere, Live Plants

Very Low

Carbon Dioxide (CO2)

Fish, CO2 Diffuser, Live Plants

Potentially High

Chlorine (C2)/ Chloramine

Water Treatment Company

Very High

Nitrogen (N2)

Atmosphere, Filter



            Now that the principle gases have been identified, the process by which these gases enter and exit the water can be discussed. Here we encounter the principle of diffusion (of which osmosis is a special category), the process by which substances flow across a gradient from an area of high concentration to that of a low concentration. In the case of water and gases, the gradient is the air-water interface.

            If there are no gas sources within a body of water, then all the gases it contains will come from the atmosphere above it (this process is illustrated in Diagram A). At the air-water interface there is a phenomena called surface tension which prevents most gases from entering the water freely and forms a membrane. Surface tension is the result of water’s cohesive properties and is an attempt by the water molecules to be as close together as possible, and thus prevent other molecules from entering or exiting. However, every once in a while a small ripple will form or the surface of the water will shift and break the bond of surface tension. This movement opens gaps in the membrane that allow gases to pass into and out of the water.


            In Diagram A, oxygen is entering and exiting the water at a rate proportional to surface area and to how many gaps are available. The point at which the gas entering the water is equal to the gas exiting the water is called the equilibrium point. Unless shifted by increasing the number of gaps in the air-water interface or by adjusting the temperature, the equilibrium point is relatively fixed. That is, a given body of water can only hold a certain amount of dissolved gas. This is especially important when more than one gas is present as one may use up most of the water’s carrying capacity and effectively block the other gas (or gases) from entering the water.



            In most aquariums there are fish, and fish produce carbon dioxide but require oxygen. This means that there is a source of one gas (the CO2) in the water but the other gas (O2) does not have a source within the aquarium (see Diagram B). The net result of such a condition is usually that the carbon dioxide, as it has an internal source, displaces the oxygen and uses up most of the water’s carrying capacity. When this happens, the oxygen level drops and the fish begin to suffocate. Ironically, if the bioload is high enough, a problem arises in that the fish could literally kill themselves via respiration.



            The answer to such a sticky problem is usually to increase the number of gaps in the air-water interface by use of a filter or aerator that agitates the surface (shown in Diagram C). When more gaps are present, the carbon dioxide can flow out of the aquarium at a rate more proportional to its production and oxygen can flow in more freely. Oftentimes, the upward shift in the equilibrium point is fairly marginal when the surface is agitated, meaning that the water does not contain much more dissolved gas in total. The more accurate portrayal is that the greater abundance of gaps allows the carbon dioxide to leave where once it could not. Consequently, if the CO2 production is especially high, the fish could still kill themselves, regardless of surface agitation. This situation also makes it apparent that no hood for an aquarium should fit too tightly for fear that the water will not be able to exchange gases at an appropriate rate.

            Aside from surface agitation, another factor that can alter the equilibrium point is temperature. As a general rule, the lower the temperature, the higher the equilibrium point. This means that cold water naturally contains more dissolved gases than does warmer water. This is especially true of oxygen in that water near freezing (32°F or 0°C) contains almost twice as much dissolved oxygen as water with a temperature of 86°F (30°C). This is paramount in that fish from colder regions are used to having rather large quantities of oxygen in their water. If cold-water (temperate) fish are kept in warm aquariums (tropical aquariums), they often become sluggish, gasp at the surface, are weakened, and die prematurely. As such is the case, it is vital that aquarists provide their pet fish with the appropriate temperature.


The Daily Cycle


            Many people are tempted to believe that gas concentrations remain constant throughout the day, but the truth is that they do not. Nonetheless, what exactly happens during the course of a day depends upon how many animals you keep and on how many live plants are present in the water. As was mentioned earlier, fish use oxygen and expel carbon dioxide. Subsequently, aquariums containing only fish have low oxygen levels during the day (when fish are most active) that correspond to high carbon dioxide levels. At night, the condition is reversed in that the inactivity of the fish places a smaller oxygen demand on the water and the fish expel less carbon dioxide.

            In planted tanks, on the other hand, the condition is reversed in that plants produce oxygen during the day and so increase the overall oxygen concentration during the day. At night, conversely, plants use oxygen for respiration just as fish do and therefore add carbon dioxide to the water (as carbon dioxide is the byproduct of aerobic respiration). Since planted tanks have a higher carbon dioxide concentration during the night, it is often prudent to turn on an aerator or turn off an operating CO2 diffuser at night so that less of the potentially deadly dissolved gas will be present. No less, if the number of fish is very small, then such precautions may not be warranted.



The Gases


            Oxygen: Necessary for all complex animals and plants, oxygen is most common as a major constituent of our atmosphere (around 20%) and as a byproduct of photosynthesis in plants. Oxygen can be displaced by other gases and thus measures must be taken to ensure that densely populated aquariums receive enough oxygen to go around.

            Carbon Dioxide: A byproduct of both animal and plant respiration, carbon dioxide often has its source directly within the aquarium. As it can be toxic, surface agitation is needed to guarantee that the occupants of tanks with high bioloads do not kill themselves.

            Chlorine and Chloramine: Both of these substances may be introduced into tap water by water treatment companies and can be extremely toxic to fish. Chlorine usually will dissipate within 24 hours if left in an open container, but chloramine is much more resilient. To rid water of either of these chemicals, commercial dechlorinators that are available at most pet shops can be used. A dechlorinator need not be the most expensive kind to work well. In fact, most generic brands work just as well as the fancier brands.

            Nitrogen: Nitrogen is inert of itself and only becomes a problem if it displaces another gas (like oxygen). Nitrogen in aquariums usually only comes from highly efficient filtration systems (like those used in saltwater applications) and is often easily removed by moderate agitation. Therefore, nitrogen is hardly ever a problem in freshwater aquariums.


            In closing this section on gases, it is important to remember three main things:

·        Agitation Helps Remove Harmful Gases

·        Temperature Determines the Oxygen Concentration that Fish Require

·        Chlorine and Chloramine Should be Removed from Tap Water Before Using that Water in an Aquarium


References for Dissolved Gases:



Ø  Levine JS. The complete fishkeeper: everything aquarium fishes need to stay happy, healthy, and alive. New York: W Morrow; 1991.

Ø  Scheurman, I. The natural aquarium handbook. New York: Barron’s; 2000.

Ø  Silberberg, MS. Chemistry: the molecular nature of matter and change. Boston: McGraw-Hill; 2006. (fourth ed.).



Water Contaminants


            “Water contaminants” is a catch-all description for all of the substances that can enter a give water supply and are either unwanted or unnecessary. Superficially, water contaminants can be broken down into two major categories: 1) those that are in the water when it reaches the consumer and 2) those that may be introduced by the consumer or the consumer’s environment. Within each of these major divisions are subdivisions that relate to the specific origin of the water or to the particulars of the final environment. This section covers the basics of how water contaminants affect fishkeeping.




            Before water reaches any of our homes, it may journey thousands of miles from its original source. As such, it is not surprising that the final product usually contains more than just water; after a long voyage, our water is bound to pick up foreign substances. Despite the ubiquity of contaminants in nearly all water sources, the exact constitution varies from region to region and sometimes home to home. No less, some sources of our water do have similar problems and that is how Keeping Fish will address the issue of water contaminants. It is an oversimplification, but not by such a degree as to render the results unusable.

            Treated Tap Water: In most cities across the United States and in other countries, the water that comes from the tap is treated by a commercial entity that renders the water “fit for human consumption.” The problem with such a mandate is that humans can deal with some things that fish cannot. For example, many cities treat their water with chlorine or chloramine which, although relatively harmless for us humans, can quickly kill some fish. Another issue is the piping through which our water flows. Water can absorb metallic ions from those pipes and such ions can be deadly to fish. Of particular interest are copper and lead as both can be extremely toxic in even minute concentrations.

            Fortunately, as mentioned previously in this book, chlorine and chloramine can be taken care of with commercially available dechlorinators. Additionally, many metallic ions can be neutralized with a metal neutralizing product. Oftentimes, dechlorinators and metal neutralizers are combined to make one product and a quick check of the labels of water treatment products at a local pet shop will reveal which products are capable of doing both (which can save the aquarist a little money in that one item is often less expensive than two). Another trick that can reduce metallic ion contamination is to let the tap run for about two to five minutes before using it to fill up the aquarium. In this way, most of the water that has been sitting in the pipes, and therefore having the highest metal contamination, will drain and be replaced by water that has not been sitting in the pipes for so long. This is especially important if the aquarist is keeping brackish or sensitive fish.

            Well Water: In more rural communities it is common for most drinking water to come from wells. Wells can be advantageous for aquarists in that they usually do not contain chlorine or other treatment chemicals unless the well owner adds them. As such, a water conditioner may not be needed for aquarists who use well water (have your well water tested before making such a determination, though).

            On the other hand, well water can contain other contaminants that can be just as problematic as those found in treated tap water. In agrarian areas, it is not uncommon for well water to contain pesticides or fertilizers used on nearby fields. Both substances can be harmful to fish. Also, well water may contain excess nitrates and phosphates which, although not necessarily lethal (unless in high concentrations), can foster unwanted algae growth in aquariums.

            The remedies for dealing with tainted well water are often less simple than those for dealing with treated tap water. The options for most well water users consist of either altering the water with a home-sized treatment unit (some water softeners and RO units can produce viable aquarium water), using water from another source, or simply dealing with the consequences within the tank. As an example of dealing with the consequences within the tank, water that has excess nitrates and phosphates can be suitable if one keeps live plants (the plants will absorb the nitrate and phosphate) and relatively tolerant fish.

            Rain Water: In the past, it was not uncommon for aquarists to use rain water in their aquariums as rain water is very soft (and therefore good for breeding projects and keeping sensitive species) and does not contain the chlorine or chloramine found in treated tap water. Presently, however, rain water has fallen out of favor as the supply of useable rain water diminishes. In many areas, pollution from factories, car exhaust, and other commercial enterprises has rendered the atmosphere unsuitable for most people, let alone for fish (rain absorbs the contaminants as it descends). Another concern is that the collection of rain water using traditional metal gutters can also taint the water. The net result is that only aquarists in very rural areas can use rain water in their tanks and only if they have a fish-safe means of collection.




            Once water has entered the aquarium it can still be contaminated from various sources. Some of the contamination can be directly attributed to the actions of the aquarist or the aquarist’s household. Other forms of contamination are more of an effect of the general environment and usually do not have a distinct source. For the purposes of this book, post-consumer contamination is divided between the two: controllable causes and uncontrollable causes.

            Controllable Causes: Some aquarists do not realize it, but anything done in the home that can affect the air can also affect the fish in that water regularly exchanges substances with the atmosphere. Subsequently, use of aerosol cans, fireplaces, pesticides, and cleaning agents can contaminate the aquarium water and kill sensitive fish. While avoiding all of these items is nearly impossible in practice, Best Practices dictate that the aquarist can help his or her fish by following these guidelines:

·      Never place an aquarium in the same room as a fireplace.

·      Always cover an aquarium and turn off the aerator (but not the filter) when using cleaning agents near or around the aquarium.

·      Avoid smoking indoors as smoke can kill some fish or stunt their growth.

·      Keep the use of aerosol sprays to a minimum and preferably confine their use to another, well-ventilated room.

·      When fumigating the home or spraying pesticides around the home, either cover the tank and shut off the system temporarily (preferably no longer than 2 hours), remove the fish to another location entirely, or cover the tank and shut off the aerator with the provision of adding activated carbon to the system to help absorb any noxious chemicals.

            Unfortunately, doing such things does not guarantee success but simply decrease the likelihood of negative results. Also, it is important to note that any time that the filtration system has been prevented from working for an extended period of time can lead system upheavals in which the filter effectually ceases to function normally. That is, the filter may be on and running but the upset the system just experienced prevents the filter from doing anything useful. As such can occur, it is suggested that system upheavals be followed by a small partial water change and a test of the water chemistry (particularly, check the ammonia, nitrite, and nitrate levels). The aquarium will need to be closely monitored for one to six weeks depending on the severity of the upset and may require frequent water changes to keep the nitrogen compounds (i.e., ammonia, nitrite, and nitrate) in check.

            Uncontrollable Causes: In keeping with the effect of contaminants in the air, there is not much an aquarist can do about whether or not a neighbor burns some waste, a guest visits the house after working with chemicals, there is a chemical spill in the area, etc. In these cases, the best remedy is usually to do partial water changes to lessen the concentration of harmful contaminants in the water. It is hard to know exactly what has entered an given aquarium, but it is a fact that changing out some of the water on a regular basis will reduce the concentrations of any harmful substances and help keep the fish healthy. Therefore, even if an aquarium does not “need” water changes for filtration reasons, it is still prudent to do partial water changes to reduce the chance of harmful contaminants building up to toxic levels.


References for Water Contaminants:

Ø  Bailey M, Sandford G. The ultimate aquarium: a definitive guide to identifying and keeping freshwater and marine fishes. New York: Smithmark; 1995.

Ø  Levine JS. The complete fishkeeper: everything aquarium fishes need to stay happy, healthy, and alive. New York: W Morrow; 1991.

Ø  Scheurman, I. The natural aquarium handbook. New York: Barron’s; 2000.



Commercial Products


            As a final statement about water chemistry, I would like to add some anecdotal comments on generally available commercial products intended for use in fish tanks. As I used to work in the pet industry, I have gleaned some knowledge from firsthand accounts regarding commercial products and would like to share that knowledge with anyone who is interested. The following comments are not intended to be authoritative and should be regarded as the net product of a singular viewpoint.


            First, my experience reinforces the old adage “if it’s not broke, don’t fix it.” I have seen many new aquarists try to fix problems that do not really exist and wind up creating more problems than they can handle. For instance, many new aquarists try to achieve a pH of 7.0. This is not so bad except that most pH values will work for keeping fish, which means that altering the pH just requires extra time and money. Also, I have seen aquarists who have been keeping fish for years go out and buy the latest “stress coat improver” despite that the fish were doing perfectly fine at the time. Simply, if the fish do not need it, then the fish do not need it.

            Furthermore, one problem frequently encountered when using commercial products is that they do exactly what they say they do. As an example, ammonia removers do indeed remove ammonia. This means that there is less of the potentially harmful substance in the aquarium but it also means that the filter, since there is no ammonia to remove, will not do an effective job if the use of the product ever ceases. In other words, the use of a product in the short term can produce long-term dependency. The same goes for products that alter pH or hardness: once you begin using them, your tank will depend upon them for stability. This does not mean that you should never alter your aquarium water. Instead, simply make a point of being aware of all the ramifications of such an alteration.

            Lastly, there is a common malady in the aquarium world in that some aquarists get so caught up in water quality that they forget more important things. As an illustration, if water is 30ppm away from the ideal hardness of a particular fish, it is unlikely that the fish is going to instantaneously die or burst into flames. However, overcrowding a tank, not feeding the fish, never doing water changes, and keeping predators with prey species can result in rather quick and untimely deaths. As such is the case, I often recommend that new aquarists spend little time on water chemistry and focus primarily on the following: cycling the tank (discussed in the next chapter), selecting compatible fish and acclimating them properly, not overcrowding or overstocking the aquarium, and frequent partial water changes and gravel siphoning.


 The above is certainly opinion and should only be taken as such.