Aquarium Stock Calculator

Check whether an aquarium is overstocked by comparing the adult length of the fish you plan to keep against what the tank and its filtration can realistically support. The page opens with a default group already in the list, so you can set the tank size and filtration and get a capacity figure and a stocking status straight away.

Two Capacity Limits and a Final Adjustment

Rather than a single rule of thumb, the calculator derives a capacity from two independent limits and keeps the lower of the two. That means a tank fails on whichever constraint is tighter — its water volume under the given filtration, or its surface area — and the result reflects the more conservative assumption.

• The volume limit takes the tank's gallons and multiplies by a filtration factor: zero point seven for basic filtration such as a hang-on or sponge filter, zero point eight five for a canister, one point zero for a canister plus powerhead, and one point one for a heavily planted tank with a canister.
• The surface-area limit is computed from a surface area of twelve square inches per gallon, then divided by twelve to give a length, which reduces to one inch of fish per gallon of tank volume.
• Because the surface limit is effectively the tank's own gallon figure, it only becomes the binding constraint when filtration is strong enough to push the volume limit above one inch per gallon — that is, in the planted configuration.
• The lower of the two limits is then multiplied by a stocking-approach factor: zero point six for a conservative approach, zero point seven five for moderate and zero point nine for heavy stocking.
• The result is reported as total inches used, a maximum recommended figure and a remaining capacity, alongside a status word.
• Status is graded in three bands: over the maximum is Overstocked, above eighty percent of it is Near limit, and anything else is Safe.

Checking a Stocking Plan

1. Enter the tank volume and choose whether it is in gallons or litres. Litres are converted at three point seven eight five to the gallon before anything else is calculated.
2. Pick the filtration level that matches your real setup. This is the input with the widest range, and upgrading from a basic filter to a heavily planted canister changes the outcome substantially.
3. Choose a stocking approach. Conservative is the safe default; move to moderate or heavy only if you are confident in your maintenance routine.
4. The list already contains one group — six tetra at one and a half inches — matching what the form displays. Change that group's type, size or quantity by editing the fields and pressing the add button again, or adjust its entry directly, then add further groups the same way.
5. Press Calculate Stocking to see the total inches, the maximum recommended figure, the remaining capacity and the status. The page measures whatever is in the list, so the default group is included even if you add nothing.
6. Read the compatibility panel underneath, which lists any aggressive or incompatible pairings among the fish you added, offers suggestions for the combination, and prints the temperature and pH range for each species.

A Twenty-Gallon Tank, Measured

Take a twenty-gallon tank with basic filtration and a conservative approach. The page opens with the default fish group of six tetras at one and a half inches adult size already in the list, so pressing Calculate immediately measures that group. This is a common beginner setup, and the verdict is worth understanding.

• The six tetras represent nine inches of adult fish, six times one and a half.
• The surface-area limit works out to twenty inches: twelve square inches per gallon gives two hundred and forty square inches, and dividing by twelve returns the tank's twenty gallons as a length in inches.
• The volume limit is fourteen inches, from twenty gallons times the zero point seven factor for basic filtration.
• The two limits are compared and the lower one, fourteen, is taken. It is then scaled by the conservative factor of zero point six, giving a maximum recommended figure of eight point four inches.
• Nine inches of fish against a maximum of eight point four makes the tank Overstocked, and the remaining capacity reads zero because the shortfall is not carried as a negative number.
• The moral is that filtration, not just volume, drives the answer. Raise the filtration to a moderate canister and the volume limit becomes seventeen inches, so the maximum rises to ten point two and the same six tetras become Safe.

Ways to Judge How Full a Tank Is

Alternative	Notes
Length-based rules	The approach this page uses, adding up adult lengths and comparing against a capacity. It is quick and widely understood, but it treats a slim tetra and a bulky fish of the same length as equivalent, which ignores a great deal of body mass and waste output.
Bioload and waste output	A more accurate lens that weighs how much waste a species actually produces. A goldfish of the same length as a slim schooling fish imposes a far heavier load, which is why the tool flags goldfish as heavy bioload and warns against mixing them with tropical fish.
Surface area for gas exchange	This page's second limit is essentially a surface-area rule, since the two are algebraically linked. It reflects the idea that the water's surface, not its volume, governs how much oxygen is available, which is why tall narrow tanks hold fewer fish than their volume suggests.
Testing the water	The most reliable signal of all is what the nitrogen cycle says. Rising nitrate between water changes, or ammonia and nitrite that will not clear, are direct evidence that the tank is over its real capacity, whatever any rule predicts. A calculator sets an expectation; the test kit settles it.

What the Verdict Does Not Cover

A stocking figure is a starting point, and several real factors decide whether a tank thrives in practice. These sit outside the calculation.

• The rules are generic and cannot see your maintenance routine. A heavily filtered tank with frequent water changes genuinely supports more fish than the formula assumes, and a neglected one supports less, regardless of what the number says.
• Adult size is taken at face value from what you enter. Understating it is easy and common — many fish sold small grow far larger — and because every group is counted by the size you type, an optimistic figure produces an optimistic verdict.
• The compatibility panel gives fixed pairings and parameter ranges rather than live analysis. It flags known problems such as aggressive or incompatible combinations and prints temperature and pH ranges, but it will not catch every subtle mismatch between species.
• Territory and swimming space are not modelled at all. Some species need a footprint rather than a volume, and a tank can be well within its inch limit and still be too cramped for a territorial fish.
• The calculation runs in the page and the fish list lives only for the session, so nothing is uploaded. A reload resets the list to the single default group rather than keeping the groups you added or edited.

Aquarium Stocking Questions

Why is the surface-area limit the same as the gallon figure?

Because the two checks are algebraically linked. Surface area is taken as twelve square inches per gallon, and the limit is that area divided by twelve inches of fish, which cancels back to the tank's gallon count. So the surface limit always equals the number of gallons. It only decides the outcome when the filtration factor pushes the volume limit above one inch per gallon, which happens in the planted configuration with a factor of one point one.

What do the three status words mean?

Overstocked means the total adult inches exceed the maximum recommended figure. Near limit means they are above eighty percent of it, so the tank is close but not over. Safe means they are at or below that eighty-percent mark. On the default twenty-gallon setup, six tetras at nine inches against a maximum of eight point four reads Overstocked; trimming to five fish would put it into the Near limit band.

How does filtration change the answer?

It scales the volume limit directly through the filter factor — zero point seven for basic, zero point eight five for moderate, one point zero for heavy and one point one for planted. A better filter raises the limit and therefore the maximum recommended figure, which is why two identically sized tanks can have very different capacities depending on what is attached to them.

Does it warn me about fish that should not live together?

Yes. Each species carries a compatibility record listing peaceful, semi-aggressive and aggressive companions, plus a temperature range, a pH range and a note. The tool checks the fish you added for aggressive or outright incompatible pairings and lists any it finds, then adds suggestions for common combinations and prints the water parameters for each species present.

Last verified: October 3, 2026

Launch Aquarium Stock Calculator

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Maintained by Huang Bin · Last updated October 3, 2026