Aquarium Stocking Calculator

The rule estimates that an aquarium can support roughly one inch of adult fish body length per gallon of water. You add up the adult length of every fish you plan to keep, then compare that total to the tank's usable capacity. This calculator starts from one inch per gallon of net water volume, then adjusts for tank size and filtration so the number is more realistic than the flat rule on its own.

First the net water volume is taken as 90% of the rated tank size, to account for substrate and decor. That volume sets a base capacity in inches at one inch per gallon. The base is then multiplied by a tank-size factor (0.75 for nano tanks up to 1.2 for large tanks) and a filtration factor based on turnover. The stocking percentage is your total fish length divided by this adjusted capacity, times 100.

A small tank has a high surface-area-to-volume ratio and far less water to dilute waste, so it offers less margin than the flat rule suggests. A large tank buffers swings in ammonia and oxygen, so it can carry slightly more fish per gallon. The calculator reduces capacity for tanks under 30 gallons and increases it for tanks of 55 gallons and up, with the 30 to 54 gallon range treated as the baseline.

Filtration sets how much waste a tank can process. Turnover is the filter's rated gallons per hour divided by the net water volume, and the hobby target is 4 to 6 times per hour. The calculator scales capacity by turnover relative to a 5x reference, capped between 0.5 and 1.5 so an oversized pump cannot inflate the estimate. Leave the filter field blank to assume standard 5x turnover.

This tool treats under 70% as understocked, 70% to under 100% as well stocked, 100% to 125% as fully stocked, and above 125% as overstocked. Overstocking strains the filter, raises ammonia and nitrate, and stresses fish. If you land above the limit, rehome fish, move up to a larger tank, or add filtration and more frequent water changes.

No. It is a planning starting point, not a guarantee. It works best for slim community fish and breaks down for tall, heavy, or messy species like goldfish, plecos, and large cichlids, where adult body mass, territory needs, and waste output matter far more than length. Always research each species' minimum tank size, cycle a new tank for 4 to 6 weeks, and stock gradually.