Water Chemistry: The Devil You Know

Water is something we certainly take for granted in this hobby and it does contain secrets that we can use to improve our craft. I see many posts where, regardless of the problem put forward, someone invariably asks for the querent's water parameters and they should. The question is, what do all those variables mean and how are they important? Let's start with some basics. Water itself is fascinating. It is sometimes called the universal solvent because of its ubiquity in the cosmos and the fact that just about everything will buffer against it. The incredible way that different substances dissolve and become salts is what makes water so perfect for life. It's a great medium for getting ions like calcium into the organism and is the basis for many biochemical reactions, the most important of those being enzyme actions, metabolism itself, transport of ions throughout the body and waste removal.

Water does this because of the electro-negativity of the lone oxygen atom in it. Oxygen attracts electrons much more strongly than the two hydrogen atoms resulting in a net positive charge. If you remember your high school chemistry then you should recall that all charges want to ground out or equilibrate.

If you can really reach back, try and remember electron shell theory where after you get past the nucleus, you have an orbit (or rather, a shell) that can only have two electrons at most (the s-shell) which is essentially helium. The next shell can have a maximum of 6 (the p-shell, which is oxygen), the next one after that can have up to 10 electrons (d-shell, titanium), then 14 (f-shell, these are the elements past 121 that are only theorized to exist) and finally 18 (g-shell, gold). I hope this isn't bringing back too many bad memories of high school chemistry but it is relevant to this discussion.

Oxygen's atomic number is eight. Its s-shell already has two electrons spoken for, leaving six to fill its p-shell. Two of those will be covalently bonded to two hydrogen atoms leaving four negatively charged electrons free to react with anything wanting to ground out. This is why water in nature is almost never chemically pure thus the nickname the universal solvent. It picks up just about everything.

Knowing all that is why water chemistry is a concern. We need to know what's in it and what those substances do to any given volume of water and a meaningful way to measure them. In our hobby there are only a few of these we need to be concerned about. There are others that can be measured but I want to stick to the basics for the moment: GH, KH and pH. They do tell us something.

General Hardness or GH

General Hardness, or GH, is the measure of cations in the water that have two valence electrons allowing them to still react with other ions or molecules. When we measure GH we are looking for calcium and magnesium, very specifically Ca⁺² and Mg⁺²_.

We're mainly concerned with calcium oxide because it is the primary compound needed by just about all living organisms to control and maintain osmoregulation. This salt is what allows the correct amount of water to diffuse in or out of the organism or its cells. Aquatic animals are a very special case.

A freshwater fish's cells if not properly regulated by the right amount of calcium oxide or other electrolytes will take on too much water and literally burst. Too much and they dessicate, thus most of our freshwater fish can't even handle brackish conditions. Similarly, a saltwater fish's cells will dehydrate if it wasn't trying to get rid of these same salts while allowing as much water as possible to diffuse across its cell membrane. This is a very simple example of how osmotic pressure works. It's a balance of electrolytes between the inside and outside of the cell allowing the right amount of water to diffuse or osmose into the cell. This balancing act in general is called homeostasis.

The unnatural glass boxes full of water and weeds we keep in our living rooms tend to run out of these hardness making ions because they are not being continually refreshed as they would in nature and our fish would suffer without water changes, a top off from the tap or the occasional shot of GH booster. Also remember that calcium is an important micronutrient for our plants and without it they can't build their cell walls in addition to having negatively impacted osmoregulation.

Because of the ubiquitousness of calcium and magnesium in water supplies around the world, it is rarely a concern and I personally have yet to read anything about someone's fish having seizures due to low electrolytes though I have seen plenty of plants suffering from a calcium deficiency. Plants need it a bit more than animals do. A calcium deficiency in plants appears as tissue necrosis. Sometimes chlorosis appears alongside it which signifies a magnesium deficiency. The two frequently happen together along with new growth that appears stunted and twisted.

GH is measured in either parts per millionth or using the German scale which just uses degrees. The tests most commonly available use a titration method. For a given volume of water one adds a single drop of reagent turning the water one color and then you count the number of drops required to make it turn into a different color, usually from red to green. That number of drops used will be the hardness as defined by the German scale.

To convert to the much more useful mg/L which is the same as ppm's, multiply the number of drops (which are the same as German degrees) by 7.143 and you will have the amount of Ca⁺² in your water column in terms of calcium oxide. For magnesium, multiply those drops by 2.019 and you'll have that component in terms of magnesium sulphate. Ideally, they should be in a 3:1 to 4:1 ratio (calcium to magnesium) and they tend towards this naturally. If that ratio is too out of whack and there is too much calcium or too much magnesium, excesses of either element will inhibit the uptake of the other. It's quite common to have a water supply that is so hard from calcium ions that plants can no longer take up available magnesium or vice versa.

Carbonate Hardness or KH

I need to make a historical note. Carbonate hardness is abbreviated KH because the German term is “Karbonathärte” from which it wholly derives.

KH is merely the measure of the alkalinity of water caused by the presence of carbonate (CO₃^2-) and bicarbonate (HCO₃^-) anions. Alkalinity is the measure of a solution's ability to neutralize acids against equivalent amounts of carbonate and bicarbonate. It's worth noting that just about all tests base their bicarbonate equivalence using sodium bicarbonate or baking soda.

KH tests work the same as the GH kits except the color change is usually from blue to yellow, the number of drops required to cause the color change being equal to the German degrees of KH. Take that number of degrees and multiply it by 17.8575 to get the mg/L or ppm's in terms of calcium carbonate. Multiply those drops by 21.8 to get the KH in terms of (sodium) bicarbonate .

Generally speaking we prefer a high KH simply because it offers more stability to the tank inhabitants as in the ability the buffer away a sudden and certainly deleterious increase of acids, usually organic acids, the products of decomposition. I'm going to stop here because KH can't be fully appreciated without an understanding of …

Negative Logarithm of Potential Hydrogen or pH

Where to start with this one... pH is a measure of the activity of bases and acids in an aqueous solution. Depending on which is more active, the solution could be said to be acidic, basic or neutral, neutral meaning that

bases and acids are equally active in our sample. It uses a scale from zero to fourteen though it is theoretically unbound so a really strong acid could be less than zero (as in a negative number) and a really strong base can be greater than fourteen. Certainly a pH of seven indicates that the solution is neutral having enough acids and bases to fully equilibrate.

GH affects the alkalinity though not by a great deal. KH is a direct measure of alkalinity and thus affects pH a great deal sending it through to the high end of the scale past neutral. If a sufficient amount of acids or an acid is added, it will send it below seven. Too much acid will cause a crash as all the carbonates are fully neutralized and only excess acids remain. It is possible to have a low pH and high GH or even a low GH and high pH but a high KH will always translate to being alkaline/basic or a low KH to being acidic.

This is not necessarily a good or bad thing as it happens commonly in nature. Think seasonal rains which are soft and acidic, collecting in a lake with a limestone bottom that is hard and alkaline. A Springtime deluge like this usually causes changes in the lake inhabitants bodies that signals them to start spawning or flowering. This is perfectly normal. In our tanks, occasionally we have to do something like this at water change with deionized or reverse osmosis water in order to get our fish to spawn depending on the species though this isn't always required.

No, a quickly crashing pH in an otherwise stable, established tank indicates an ecological disaster. Because our tanks are stand alone units that only get refreshed when we do a water change or filter maintenance, a falling pH must be recognized quickly and corrected. Excess decomposition is the culprit the vast majority of the time and the cycle is almost self feeding: An acidic environment makes it easier for material to decompose which only makes things more acidic which eats up the available oxygen until the lock of oxygen forces the bacteria that do the decomposing to switch to an anaerobic mode which makes things go from bad to stinky, acidic worst with a high mortality rate.

If you ever experience your fish suddenly gasping, unusual odors emanating from the tank, the water color suddenly changing and sores or white crud spontaneously forming on your fish this is an oxygen crisis brought on by a pH crash. It's still a question of which came first, the chicken or the egg but that can be thought about later. You need to do an immediate, 80% or greater water change straight from the tap. Try and save a sample to test later. It will likely indicate very soft, acidic conditions that were not there before or way out of what would be considered normal for you. In fact, regardless of the root cause, anytime you suspect a problem, do a water change and double up on your dechlorinator of choice just as standard practice.

There really isn't a lot in the literature I've seen about possible problems with a quickly spiking pH. I could see it happening if someone poured a lot backing soda in your tank but that's about it really. Over time, tanks tend towards acidification as organics accumulate between water changes. It could be a real though rare problem but I've not heard of it so we'll let this scenario remain fallow for the meantime.

Now, some interesting things do happen that are unique to acidic tanks versus basic tanks. In fact I touched on one such generalization earlier and given what we've just explored, I think you'll see how these relationships exist and find that these are very safe, technically sound general assumptions. Let me just shoot through them in list format.

1. Higher concentrations of oxygen tend to be found in hard, basic water. Even more if that water is cool.

2. You can host a lot of nitrifying bacteria if you can maintain the above conditions but keep the water rather warm.

3. Ammonia gets built into the slightly less deadly ammonium in soft, acidic water which is a great boon to plants and ammonia oxidizing bacteria.

4. Plant nutrients in general are more easily translocatable in acidic environments and can be pulled from the water column more readily.

5. Medicines, when required, are more effective in an acidic environment and fish have a tendency to not get sick as frequently. Anecdotally, acids like tannins are like a tonic.

6. Algae also seems to have a harder time getting established in an acidic tank though that is more anecdotal than anything.

7. Hard, alkaline tanks tend to be more stable over the long term.

8. Nitric acid, a natural product of ammonia oxidation is very quickly neutralized into nitrate in hard, alkaline water.

9. Ammonia is both a weak acid and a weak base.

The list goes on and on but these are the neater ones I wanted to share with you all. Consider them as some cool bits of trivia.

The Admonition

At this point, I feel I've summarized everything you reasonably could need to know about the most basic facts of water chemistry as applied to planted aquaria in general. For those of you who do not raise or propagate certain rare, delicate, wild caught or otherwise sensitive species of plants and animals, I want to ask you to forget you have every read this article or that any of it matters in theory or practice outside of troubleshooting where time is not of the essence or is just the result of mere scientific curiosity. Put it out of your mind right now and go about your life happily enjoying your tanks for what they are.

I excused a certain class of individuals, like the growers of Pipeworts or breeders of Discus because they have a very real need to understand these factors and keep their arguably rare and delicate fish and plants happy, healthy and productive so that those who want them may continue to enjoy these challenging organisms.

No, most of our fish are line bred in liquid concrete and the vast majority of our plants are merely opportunistic weeds whose only requirement is that their water be wet (thanks Rex!). I've seen Apistogrammas and wild caught Altum Angelfish breeding successfully in what could be called limewater and Cryptocoryne becketii invading the limey riverbanks of Central Texas.

The only common fish that I can think of off the top of my head that hail from the soft, acidic black waters of South America are Cardinal Tetras (Paracheirodon axelrodi) and Rummy Nose Tetras (Hemigrammus rhodostomus) and even then that fact only matters if you intend to breed them. I'm sure there are others, but these two stand out in my memory. If you're curious, too long of exposure to the calcium ions in hard, alkaline water renders them sterile. In Rummy Noses, it's irreversible but with Cardinals, it is reversible once they are returned to black water conditions.

Don't go messing with your water chemistry. It's expensive, mostly unnecessary and subjects the fish and plants to needless stress from rapidly changing conditions brought on by the tanks inability to buffer. Be happy with your tap water and just enjoy your fish and plants. Do not go chasing the dragon and that is exactly what mucking about with your water chemistry is. Unless there are obvious signs of deficiency that not even a water change or your fertilizer regime can address and everything else is fine, it's not worth it. Just review your husbandry and seek out help. Believe me, the source of water problems and unstable environment has more to do with the aquarist than anything else.

Preemptive Strike

Someone is going to bring this up sooner or later so I want to kill it before it does. Have you ever seen this chart?