Fish stocking calculator

How many fish can your tank hold? Enter your tank size and add the fish you want. You'll see how full the tank would be, plus warnings about groups, temperatures and fish that don't get along.

Tank size (US gallons)

Don't know the size? Use the aquarium volume calculator.

Fish or invertebrate

Count

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This is a rough guide based on adult fish size and how much waste each kind makes. Good filtration and regular water changes matter just as much.

How many fish can I put in my tank?

The old rule is "one inch of fish per gallon", measured at the fish's adult size. It's a decent starting point for small, slim fish like tetras and rasboras, but it breaks down for big or messy fish. One 8-inch goldfish makes far more waste than eight 1-inch tetras.

This calculator adjusts for that. Each fish counts by its adult length multiplied by how much waste it produces, so goldfish and plecos count for more and shrimp and snails count for very little.

What the results mean

Under 85%: good. Room to spare, and the easiest tank to keep stable.
85–100%: fully stocked. Fine with a good filter and weekly water changes.
100–115%: slightly overstocked. Possible with extra filtration and frequent water changes, but not recommended for beginners.
Over 115%: overstocked. Expect ammonia problems, stressed fish and algae.

It's not only about numbers

A tank can have plenty of room and still fail if the fish don't match. Schooling fish get stressed in small groups, some fish nip long fins, big fish eat small ones, and cold-water fish can't live with tropical fish. The warnings above check for these.

Add fish slowly

Even in a cycled tank, add a few fish at a time and wait one to two weeks between additions. This gives the good bacteria time to catch up with the extra waste.