Fish Tank Stocking Calculator

Whether a species fits your tank, judged on adult size, footprint and group requirements — not on the inch-per-gallon rule, which does not survive contact with the arithmetic.

Last reviewed August 2026 · Free · Nothing you enter is stored

Why one inch of fish per gallon does not work

The rule is easy to remember, which is the only thing it has going for it. The arithmetic falls apart on the first assumption, which is that a fish twice as long is twice as much fish. It is not. Body mass scales with roughly the cube of length, so a two-inch fish carries about eight times the mass of a one-inch fish of similar shape. Adding lengths together and comparing the total to a gallon count therefore misprices the bioload badly, and it misprices it in the direction that hurts — the larger fish are far heavier than the rule believes.

The second assumption is that waste production tracks body length. It does not track body mass either. Small fish are metabolically more active per gram than large ones, so the relationship between size and waste output is a curve rather than a straight line running through the origin. Any rule expressed as a single multiplication is going to be wrong somewhere along that curve, and the inch-per-gallon rule is wrong at both ends of it.

Then there is the failure that causes the most actual harm, which has nothing to do with metabolism. The rule says nothing about adult size. A common pleco sold at two inches occupies two inches of the budget on the day it is bought and twenty-four inches of it three years later, in a tank that never changed. This single omission is responsible for more rehoming pleas and more quietly euthanised fish than any error of arithmetic.

The rule also has no view on behaviour. A two-inch tetra and a two-inch cichlid are not interchangeable, because one shoals peacefully in open water and the other claims a patch of the bottom and defends it. And it ignores surface area, which is what actually limits gas exchange — oxygen in, carbon dioxide out, all of it happening at the top of the water column and none of it in the middle.

The 1960s rule was better than the one that replaced it

The version of this rule that circulated before the volume one was based on surface area. It allowed roughly one inch of fish per twelve square inches of water surface for tropical species, and one inch per twenty-four square inches for coldwater fish, which need more surface because cold water holds oxygen differently and the fish concerned are usually larger and more active.

That rule measured something real. Gas exchange happens at the surface, so a footprint-based budget is at least pointed at the correct constraint, and the tropical-versus-coldwater split shows that whoever wrote it had thought about the problem. It also explains why a long, shallow tank has always supported more fish than a tall column of the same volume.

The volume-based rule came later, and one inch per gallon looks like a garbled merger of the two. On a standard ten-gallon tank the two rules produce similar-looking answers, which is probably how the substitution went unnoticed. What was lost in the swap was the reasoning. A gallon count says nothing about the shape of the water it describes, and shape is exactly what the original rule was measuring. Our volume calculator covers the same footprint point from the other direction.

What serious keepers use instead

The approach that has replaced the inch rule among people who keep fish well is unglamorous: a minimum tank size and a minimum footprint per species, published individually, derived from adult size and from how the animal behaves. Seriously Fish and the better care-guide publishers work this way, and it is what this calculator does.

It answers questions in the right order. Does this animal fit in this box when fully grown, in a tank of this shape, in a group of the size it needs. Only once those are settled does bioload arithmetic become interesting, and by then the filtration and water change schedule are doing most of the work anyway.

Minimum tank size is a floor rather than a target. A species listed at thirty gallons is not comfortable at thirty and miserable at twenty-nine; the figure marks the point below which the keeper is knowingly compromising. Above it, more room is almost always better, and more floor space is better than more depth.

Sold small, grows enormous

The most expensive stocking mistakes are not overstocking a tank by a few fish. They are buying one animal that was never going to fit. These are the species in our data most often sold to people whose tanks cannot house them as adults.

The common pleco is the archetype. It is sold at two or three inches as an algae solution, reaches two feet, needs 180 gallons or more, stops eating algae long before then, and is nearly impossible to rehome because every fish shop in the country already has three. A bristlenose pleco does the same job at four and a half inches in a twenty-gallon tank, and there is no version of this decision where the common pleco is the better one.

Clown loaches fail differently. They are peaceful, long-lived and genuinely need company, so the tank has to house a proper adult group rather than one fish — which is how a ten-inch species arrives at a requirement of around 170 gallons. Bala sharks are the same shape of problem at thirteen inches and 125 gallons, and they are routinely sold for twenty-gallon tanks.

Then there are the two that are not home aquarium animals under any framing. The iridescent shark passes four feet and eighty pounds; the red-tail catfish reaches four feet and the same weight, and it is a predator throughout. Public aquariums receive offers of both constantly and decline nearly all of them. If a shop sells you one, the shop is wrong.

Goldfish belong in this section too, for a reason that has become a cliché without becoming less true. A fancy goldfish reaches seven inches and produces a heavy bioload; a common goldfish reaches a foot or more and is a pond animal. Neither has any business in a bowl, and the bowl is not a smaller version of a correct home — it is an unfiltered, unheated container in which the waste has nowhere to go.

Group size is part of the stocking question

For shoaling species the number of fish is a welfare requirement rather than a spare parameter. Tiger barbs are famous fin-nippers below about eight fish and unremarkable community residents above it, and the behaviour that gets them their reputation is a symptom of being kept in threes. Corydoras kept alone stop foraging and hide. Rasboras and tetras kept in twos spend their time near cover rather than in the open, which is both a poorer display and a worse life.

This changes how a small tank should be stocked. The instinct is to buy one of several attractive species, which produces six lonely fish in a tank that could have held a proper group of one of them. A ten-gallon tank with eight ember tetras is a better tank in every respect than a ten-gallon tank with two embers, two neons and a pair of guppies, and it is easier to keep.

A few species in the table want the opposite treatment. Male bettas and male paradise fish must be kept apart from their own kind. Mbuna such as the yellow lab are deliberately crowded, because dispersing aggression across many targets is what keeps a Malawi tank peaceful, and normal stocking logic genuinely does not apply to them.

Full species reference

Adult size, minimum tank size, temperature range and group requirement for all 46 species in our data. Any of these can be selected in the dropdown above.

Adult sizes, minimum tank sizes, temperature and pH ranges are drawn from Seriously Fish species profiles and Aquarium Co-Op care guides. Where sources disagree the more conservative figure is used, because the disagreements are almost always about fish that outgrow what they are sold for. Minimum tank size is a floor, not a target.

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