Quick Answer
Aquarium stocking has two published rules and they disagree. The inch rule allows 1 inch of fish per US gallon; the surface rule allows 1 inch per 12 square inches of water surface for slim-bodied fish, or per 20 for wide-bodied ones. On a 20 long the surface rule allows 30.0 in of fish against the inch rule's 18.7 in, a 60% gap, while on a 75 gallon the inch rule is the more generous of the two. Both rules count length, but oxygen demand follows body mass, roughly length cubed — so six 1 in fish and one 6 in fish are identical to both rules while the single fish carries 36 times the body mass.
What this calculator does
There are two long-standing rules for stocking a freshwater aquarium, and they do not agree. This calculator runs both on your tank and shows the gap, then shows the correction that neither rule makes. It does not pass or fail your stocking — no arithmetic can, because territory, aggression and swimming style are not numbers.
Surface rule: 1 inch of fish per 12 in² of water surface (slim-bodied), or per 20 in² (wide-bodied)
The surface rule exists because oxygen enters the water at the surface, not throughout the volume. Two tanks holding the same water can have very different surfaces, so the two rules can give very different answers for the same tank.
How to use it
- Pick the tank shape and enter the footprint. The dimension fields change to match the shape you choose.
- Enter the water depth — surface down to the substrate, not the full height of the glass. This sets the volume the inch rule uses.
- Enter each species’ ADULT length. This is the step that decides everything, and it is the one people get wrong. A fish sold at 3 cm may finish at 12 cm.
- Set the body type. Slim covers tetras, danios and rasboras. Wide covers gouramis, angelfish, cichlids and anything deep-bodied or thick through the middle.
- Add up to four species with the button, then read the two rules side by side.
If you do not know your tank’s volume, the aquarium volume calculator works it out from the same dimensions, including the water you lose to substrate and the rim gap.
The two rules on standard tanks
Every row is a standard US glass tank with slim-bodied fish. “Allowed” is total fish length.
| Tank | Surface | Volume | Inch rule | Surface rule | Gap |
|---|---|---|---|---|---|
| 10 gallon | 200 in² | 10.39 gal | 10.4 in | 16.7 in | 60% |
| 20 long | 360 in² | 18.70 gal | 18.7 in | 30.0 in | 60% |
| 29 gallon | 360 in² | 28.05 gal | 28.1 in | 30.0 in | 7% |
| 40 breeder | 648 in² | 44.88 gal | 44.9 in | 54.0 in | 20% |
| 55 gallon | 624 in² | 56.73 gal | 56.7 in | 52.0 in | 9% |
| 75 gallon | 864 in² | 78.55 gal | 78.5 in | 72.0 in | 9% |
The rules cross over. On the shallow, wide tanks the surface rule is 60% more generous. On the 55 and the 75 it is the inch rule that allows more, because those tanks buy their volume with height rather than footprint. The 20 long and the 29 gallon are the clearest case: identical 360 in² surface, so identical surface-rule allowances, while the 29 holds half as much water again.
Why both rules understate a big fish
Both rules count total length. Length is the wrong unit, because a fish’s oxygen demand and waste output follow its body mass, and mass scales roughly with the cube of length. This is not a nuance. It is the single largest error in both rules.
| Stocking | Total length | Surface cost | Body mass |
|---|---|---|---|
| Six 1 in fish | 6 in | 72 in² | 6 units |
| Three 2 in fish | 6 in | 72 in² | 24 units |
| Two 3 in fish | 6 in | 72 in² | 54 units |
| One 6 in fish | 6 in | 72 in² | 216 units |
All four stockings are identical to both rules: 6 inches of fish, 72 in² of surface. The last one carries 36 times the body mass of the first. As Aquarium Science puts it, a two inch fish does not need twice the aeration of a one inch fish — it needs about eight times as much.
The calculator reports this as body-mass units, where one unit is a 1-inch fish. It also names the share carried by your largest fish, which is usually the number that surprises people.
A worked community tank
A 20 long, 30 × 12 in with 12 in of water — 360 in² of surface and 18.70 US gallons. Stock it with eight 1.5 in slim fish, six 2 in slim bottom-dwellers and one 4 in wide-bodied fish:
- Total length is 28.0 inches.
- Surface rule: the slim fish cost 12 in² per inch and the wide one costs 20, giving 368 in² against 360 available — 102%.
- Inch rule: 28.0 inches against 18.7 gallons — 150%.
- Body mass: 139 units, of which the single 4 in fish is 46% — on 14% of the total length.
Two rules, two answers, and a third view that says the one fish you were least worried about is nearly half the load. That is the honest state of aquarium stocking arithmetic, and it is why no calculator should hand you a yes or no.
Tank shape matters more than volume
Two tanks holding exactly the same water, 14.96 US gallons each:
| Tank | Volume | Surface | Surface rule | Inch rule |
|---|---|---|---|---|
| Tall, 12 × 12 × 24 in | 14.96 gal | 144 in² | 12.0 in | 15.0 in |
| Flat, 24 × 12 × 12 in | 14.96 gal | 288 in² | 24.0 in | 15.0 in |
The inch rule cannot tell these apart. The surface rule gives the flat tank twice the allowance, which is why tall hexagonal and column tanks are harder to stock than their gallon rating suggests. It is also why footprint, not volume, is the number experienced keepers quote first.
Slim versus wide bodies
The surface rule charges a wide-bodied fish 20 in² per inch instead of 12. On the same 20 long:
| Body type | Surface cost | Allowed on a 20 long |
|---|---|---|
| Slim — tetras, danios, rasboras | 12 in² per inch | 30.0 in |
| Wide — gouramis, angelfish, cichlids | 20 in² per inch | 18.0 in |
A tank of wide-bodied fish gets 40% less allowance than the same tank of slim ones. Mix the two and the calculator charges each species at its own rate rather than averaging them.
What the rules do not know
Both rules are about waste and oxygen. Neither models anything else, and the gaps matter more than the arithmetic:
| Not modelled | Why it changes the answer |
|---|---|
| Shoaling minimums | Many small species need groups of six or more to behave normally, regardless of capacity |
| Territory | Two fish that each fit may still not fit together |
| Surface agitation | A still surface does roughly 5% of the gas exchange of a broken one |
| Filtration and maintenance | Turnover and water-change frequency shift the real ceiling in both directions |
| Swimming style | Active open-water swimmers need length of tank, not litres |
What to do when a rule says you are over
Being over one rule is a direction, not a sentence. Which rule you are over tells you what to change, and the three problems have different fixes.
- Over the surface rule but not the inch rule — you are short of gas exchange, not water. Break the surface properly with the filter outlet or an air stone before anything else. A still surface does roughly 5% of the exchange of a broken one, so this is the cheapest capacity you will ever find.
- Over the inch rule but not the surface rule — you are short of water for the waste being produced. More frequent or larger water changes address this directly; a shallow, wide tank is the usual cause.
- One fish dominating the body-mass figure — neither of the above helps much. A single fish at 46% of the load will keep that share no matter how the rest is arranged, and the honest fix is a bigger footprint or a different fish.
- Over both rules — the arithmetic has stopped being the interesting question. Re-check the adult lengths first, because an over-estimate there is more common than genuine overstocking.
The order matters. Surface agitation is free and immediate, water changes are cheap and ongoing, and tank size is neither — so work down that list rather than starting at the bottom.
Substrate, volume and the numbers that feed this one
The volume this calculator uses is water volume, so substrate matters: a 5 cm bed in a 20 long displaces 11.61 litres, which is water the inch rule no longer gets to count. If you are setting the tank up, the aquarium substrate calculator turns a bed depth into kilograms and bags, and the volume calculator subtracts the same bed to give the net water figure to enter here.
The published figures used throughout come from long-standing aquarium guidance — the inch-per-gallon and 12/20 in² rules are set out in Fish Tanks Direct’s swimming-space guide, and the cube-law criticism of them comes from Aquarium Science, linked above.
Frequently asked questions
Whichever gives the smaller number for your tank, and then treat it as a ceiling rather than a target. On shallow wide tanks that is usually the inch rule; on tall narrow tanks it is the surface rule. Where they disagree by 60%, that disagreement is the honest answer.
It is a rough floor for small slim fish and it breaks badly for large ones, because it counts length while the biology follows mass. Six 1-inch fish and one 6-inch fish are both “6 inches” to the rule, and the single fish carries 36 times the body mass.
Adult size, always. Stocking calculated on shop-size fish is the most common way a tank becomes overstocked without anything being added to it.
Anything deep through the body or thick through the middle — gouramis, angelfish, most cichlids, goldfish. Tetras, danios, rasboras and similar torpedo-shaped fish are slim. The rule charges wide bodies 20 in² per inch instead of 12.
Because the water below the surface is what the inch rule counts, and most tanks run a few centimetres below the rim with a substrate bed underneath. Entering the full glass height overstates the volume and therefore the allowance.
It changes the waste side, not the oxygen side or the space side, and neither rule models filtration at all. A filter cannot create surface area or swimming room, so treat extra filtration as insurance rather than as extra capacity.
That is a judgement, not a calculation, which is why this tool reports both rather than issuing a verdict. What the numbers can tell you is which constraint you are hitting — surface, volume, or one large fish dominating the load.
A way of comparing stockings that length cannot. One unit is a 1-inch fish; a fish of length L counts as L³ units. It is a relative scale for seeing which fish dominate your bioload, not an absolute measure of anything.
