Aquarium Stocking Calculator – How Many Fish in My Tank

How many fish can I put in my tank? It is the first question every new fishkeeper asks, and the old inch-per-gallon rule answers it badly. It ignores temperament, body shape, waste, and swimming space, so it overstocked some tanks and understocks others. This free aquarium stocking calculator uses your tank litres, adult fish size, and temperament to give a more sensible fish count.

Tank size in litres is the honest starting point, because fish live in water volume, not in glass length alone. A tall narrow tank and a long shallow tank can hold the same litres yet suit very different fish, so volume plus temperament together give a fairer estimate than any single rule. Adult size matters too, since a two-centimetre tetra and a ten-centimetre cichlid are not interchangeable units.

Using the calculator takes a minute. Enter your tank volume in litres, the adult size of the fish in centimetres, and the temperament from peaceful to aggressive. Press Calculate stocking and you get a sensible fish count, the litres allowed per fish, and the total litres used. Aggressive fish get far more litres each, because territory matters as much as waste.

A worked example shows the logic. A one-hundred-litre tank with five-centimetre peaceful fish allows ten litres per fish, giving ten fish. Change the temperament to aggressive and each fish needs thirty litres, giving only three fish in the same tank. The same water, the same fish size, and a very different answer, which is exactly why temperament belongs in the calculation.

Stocking is a ceiling, not a target. A tank at eighty percent of its sensible count is usually healthier, easier to maintain, and more pleasant to watch than a tank filled to the limit. Plants, good filtration, and regular water changes raise what a tank can handle, while weak filtration, overfeeding, and infrequent changes lower it. Treat the result as a planning figure, not a permission slip.

Add fish slowly, a small group at a time, with a week or two between groups. The filter bacteria need time to grow to match each new load, and adding a full stocking at once often causes an ammonia spike that harms or kills fish. Quarantine new fish when you can, and watch the first group for a week before adding the next.

Schooling fish break the simple count in a good way: tetras, rasboras, and similar species need groups of six or more to feel safe, so a count of three is wrong even when the maths allows it. Bottom feeders, algae eaters, and centre swimmers also use different zones, so a mixed community can hold a few more fish than a single-species tank of the same volume, as long as each group has its minimum numbers.

This calculator gives an estimate for planning, not a guarantee for any particular species. Always check the adult size, temperament, and group needs of the exact fish you plan to keep, and ask a good fish shop to sense-check your list. It is free, it needs no sign-up, and it runs entirely in your browser.

Filtration capacity is the hidden factor that most stocking guides skip. Two filters with the same flow rate can hold very different amounts of bacteria, because bacteria live on surfaces: sponge, ceramic rings, and other media. A filter stuffed with media supports more fish than a filter with a single thin sponge, even when both boxes claim the same litres per hour. When you buy a filter, check the media volume, not just the flow number, and never replace all your media at once, since that throws away the bacteria colony you spent weeks growing. Rinse media in old tank water during water changes, and replace it a little at a time. A mature, well-stocked filter is worth more than a bigger number on a box, and it is the reason experienced keepers can safely hold a few more fish than a beginner with the same tank.

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