The Fishsheets vs. Other Common Stocking Rules

Why not use the inch-gallon-rule or other stocking guidelines?       
Sometimes people ask me why I use the Fishsheets in place of other stocking guideleines, and I admit that using an intricate spreadsheet program can sound pretty overly-elaborate unless you are aware of the shortcomings of the more common guidelines. When people ask me why I choose such an extreme means of assessing my aquariums, I usually respond by asking them to look over this list of "stocking rules" and tell me which one is correct:

Inch per Gallon Rule

            Description: Probably the most popular stocking rule. This rule states that for the average tropical (warm water) aquarium, you can stock the tank with one inch of fish length per gallon of water in the tank. Tends to work best for fish between one and six inches long.

            Example: A ten-gallon tank can, by this rule, can hold ten one-inch fish, five two-inch fish, or two five-inch fish.


Graduated Inch per Gallon Rule

          Description: Extends scope of original inch per gallon rule by stating that temperate fish need two gallons of water per inch of fish and koi (a large type of ornate carp similar to the common goldfish) need three gallons per inch of fish.

            Example: A ten-gallon tank can house ten one-inch tropical fish, five one-inch temperate (cold water) fish, or three one-inch koi.


20 Sq. In. per Inch of Fish

          Description: Older rule (at least 1970’s) based on the premise that since the surface of the aquarium is responsible for oxygen and carbon dioxide exchange then stocking capacity should be proportional to surface area. This rule states that for every twenty square inches of surface area, you can keep one inch of tropical fish length.

            Example: A ten-gallon tank with a surface area of 200 sq. in. (10 in. ´ 20 in.) can house ten one-inch fish or five two-inch fish.


Graduated Area Rule

          Description: Created by a biologist in the early 1990’s (Joseph Levine). Modern analog to the twenty square inches per inch rule. This rule has six conditional statements for freshwater systems and are as follows:


Fish Size                                              Tropical                                   Temperate

Fish 2” or Less                                      6 in.² / in. Fish Length                12 in.² / in. Fish Length

Fish Over 2” and Less Than 4”                9 in.² / in. Fish Length                18 in.² / in. Fish Length

Fish Over 4”                                          12 in.² / in. Fish Length               24 in.² / in. Fish Length


            Example: A ten-gallon tank with 200 sq. in. of surface area can house 16 two-inch tropical fish, 8 two-inch temperate fish, 4 four-inch tropical fish, or 2 four-inch temperate fish.
Most people, after looking over the list, begin to see why I created my own stocking rules: the above stocking guidelines do not agree with each other. Some of the above rules denote that volume is the most important consideration while others use surface area. Additionally, the rules have a very specific range of convergence; they were made for certain sizes and types of fish and omit all other possibilities. For example, if an aquarist asks him/herself, "What is the stocking rule for a semi-tropical loach measuring 12 inches long?," then that aquarist will not receive an answer from the above list for two major reasons: First, none of the above rules deal with semi-tropical fish. We could guess that the appropriate stocking level would be somewhere between the temperate and tropical rules, but where specifically? Second, none of the above rules deal with fish that are 12 inches long. Yes, some of the rules address fish over a certain size, but those rules lump all the larger fish together despite that those fish would probably have different metabolisms and habits.
Of course, issues such as semi-tropical fish and specific size are only part of the problem not addressed by the above rules. Some other important facets of aquarium husbandry are filtration and the appropriate aquarium size. None of the common rules address either the necessity of having a filter nor ensuring that the tank is actually big enough for the proposed fish. As an illustration, consider a little 20-gallon aquarium with an air-driven UGF (Under-Gravel Filter). According to most of the guidelines mentioned, placing 14 inches of fish in the tank is not going to be a problem--or is it? If those 14 inches are represented in the form of one Oscar (type of large cichlid), then there is going to be significant problems. First, an adult Oscar can easily weigh more than a pound and excrete a large amount of waste. Subsequently, the little UGF will not be able to keep the water clean and the aquarist will be forced to perform water changes on a nearly daily basis. Second, a standard 20-gallon tank is only 12 inches wide, meaning that the poor Oscar will not be able to turn around. Unfortunately, filtration and aquarium size considerations are life-or-death factors but are overlooked by most of the common stocking guyidelines.
Succinctly, general rules are just that--general. No aquarist that I have met has a general aquarium; everyone owns a unique system with unique needs. Consequently, I have developed a set of equations that "fills in" the gaps left by the common stocking guidelines so that aquarists can have definite answers to less well-defined situations. My computations are not perfect, but they do get aquarists much closer to safe results than any other rule or guideline I have seen.
So why spreadsheets? Well, I want everyone to have the best chances of success, but I am not blind to the fact that accurate guidelines are intricate beyond the means of most people to memorize. Not even I have memorized all of the formulas that are in my stocking spreadsheets and I do not want anyone else to have to worry about all those formulas. Spreadsheets are simply handy tools in that they can contain large volumes of information that becomes available with the click of a mouse.