Aquarium Stocking Calculator
Enter your tank size and the fish you plan to keep. The calculator checks stocking level, minimum tank size, school sizes and compatibility of every pair, and suggests a heater setting that suits everyone.
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.