How the stocking level is calculated
Each fish starts from its adult length, scaled up for body mass, because a fish twice as long weighs and eats several times more. Each species then gets a bioload factor: slim, active fish like tetras count as 1, heavy or messy fish count more (plecos 1.5, fancy goldfish 3), and shrimp and snails very little. The total is compared with your tank's gallons, adjusted for filtration. Up to about 90% is a comfortable, well-stocked tank. Above 110% is overstocked for most keepers.
The calculator also checks the things that matter as much as numbers: whether a fish needs a bigger tank to swim properly, whether a schooling fish has enough friends, and common clashes like tiger barbs with long-finned fish.
Stocking guides by tank size
- How many fish in a 5 gallon tank
- How many fish in a 10 gallon tank
- How many fish in a 20 gallon tank
- How many fish in a 29 gallon tank
- How many fish in a 55 gallon tank
Frequently asked questions
Is the one inch per gallon rule accurate?
Only for small, slim fish. It ignores body mass: a 6 inch fancy goldfish produces many times more waste than six 1 inch tetras. The calculator scales for body mass and each species’ bioload, and checks minimum tank sizes.
How fast can I add fish to a new tank?
Cycle the tank first (4 to 6 weeks, or use a fishless cycle with ammonia), then add a few fish at a time, waiting 1 to 2 weeks between additions so the bacteria can catch up.