Aquarium Stocking Calculator

We start from your gross tank volume and apply a usable-volume correction of about 85%, because substrate, decor and the gap below the rim mean the water your filter actually processes is less than the label says. That corrected volume sets the system's bioload capacity, scaled by your filtration level.

Each species then contributes load units based on its adult size — raised to a superlinear exponent, because waste output grows faster than body length — normalized against a small-tetra baseline. The fill percentage you see is total load against capacity: 100% means the estimated bioload matches what the volume and filtration can process.

Separately, every pair of species you add is checked for temperature, pH and hardness overlap, temperament, adult-size ratio, and traits like fin-nipping — plus a list of documented species-specific exceptions (a betta next to flowing guppy fins, barbs with slow long-finned fish). All values come from published references with the sources stored per species. These are estimates for planning, not guarantees: cycle the tank first and keep monitoring water parameters.

Ask how many fish fit in a given tank and you'll hear rules of thumb: an inch of fish per gallon, a centimeter per liter, one small fish per ten liters. They survive because they're easy — not because they work. All of them collapse the real question, which is how much dissolved waste your particular water volume and filtration can process before nitrogen compounds accumulate faster than your maintenance removes them.

A stocking calculator answers that question with arithmetic instead of folklore. It corrects your gross volume down to the water that's actually there, converts every species into an estimated waste load based on its adult mass rather than its length, and compares the total against the processing capacity of your filtration. The difference shows up immediately in mixed communities: a school of a dozen small characins barely moves the needle, while a single medium cichlid can claim a third of a tank's capacity by itself — something no per-inch rule can express.

Headcount math alone still isn't a stocking plan, though. A tank at a comfortable 60% fill can be a disaster if the fish in it can't coexist: parameters that don't overlap force one species into chronic stress, a boisterous nipper turns a slow veiltail's life short and miserable, and yesterday's cute juvenile grows into a predator whose tankmates fit in its mouth. That's why this page runs a pairwise compatibility check alongside the fill percentage and shows the specific reason for every warning — a temperature gap, a size ratio, a documented behavioral conflict — so you know what to change, not just that something is wrong.

Use the number as a planning ceiling with margin, not a limit to reach. Stock gradually, cycle the aquarium fully before the first fish, and let the tank itself cast the deciding vote: published data describes species averages, and your individual fish haven't read the literature.