Aquarium Stocking Calculator
Pick your tank size and the fish you plan to keep. We check stocking level, minimum tank size, school sizes and every pair's compatibility, and suggest a heater setting that suits everyone.
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Your tank size
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Your fish
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Result
Tank size guides
5 gallon · 10 gallon · 20 gallon · 29 gallon · 40 gallon · 55 gallon · 75 gallon
FAQ
How is the stocking level calculated?
Each fish counts as units: one slim 2 inch (5 cm) fish = 1 unit, scaled by (adult length ÷ 5 cm)^1.5 × a body-mass factor. A well-filtered tank holds about 1 unit per US gallon. Under 85% is comfortable; 85–100% needs strong filtration and frequent water changes; over 100% is overstocked.
Does it account for filtration and plants?
No. It assumes a normal, well-maintained filter. Heavily planted tanks with strong filtration can hold somewhat more, but we deliberately stay conservative.
Note: Compatibility depends on individual temperament, tank size, decor and how fish are introduced. Our verdicts use conservative rules applied to published care data (see methodology). They are guidance, not a guarantee. Quarantine new fish and watch for chasing, torn fins or hiding.