“How many fish can I keep?” is the wrong first question. The right one is “what size tank do these fish need?” Pick the fish you want below and the calculator gives you the gallons and a recommended tank size, then flags the combinations that cause fighting, stunting, or dead fish.
Aquarium Stocking Calculator
Pick the fish you want and how many. The calculator tells you the tank size they need.
Estimates for freshwater community tanks, not a hard limit. Real capacity depends on filtration and water-change habits. When unsure, choose the larger tank and over-filter.
How the calculator works
Every species carries a weighting for its adult size and waste output, not just its length. Three fancy goldfish need far more water than three neon tetras, even though goldfish are only a few inches bigger, because they eat and pollute like tanks twice their size. The tool adds those loads, applies each fish’s minimum tank footprint, and returns the larger of the two numbers. Then it checks the mistakes beginners make most: schooling fish kept in twos and threes, cold-water and tropical fish mixed together, fin-nippers housed with long-finned tankmates, and territorial cichlids crammed into one tank.
Read the warnings, not just the number
The gallon figure is a floor, not a target. A schooling fish like a rummynose tetra needs a group of six or more to feel safe, so the calculator budgets for the whole shoal. And filtration is the real ceiling: aim for a filter rated at four times your tank volume per hour. If you are still choosing gear, the guide to aquarium filter types explains how to match one to your stock. For every tank number in one place, see all our aquarium calculators.
Before you add any fish
The tank size only matters once the water is safe. A new tank has no bacteria to process waste, so a full stock on day one triggers an ammonia spike. Work through cycling the tank first and follow the full setup walkthrough before adding livestock. For a worked example, see how many fish actually fit in a 10-gallon tank.
Common questions
Is the inch-per-gallon rule real? No. It ignores body shape and waste output. A 10-inch pleco is not the same as ten 1-inch tetras. This calculator weights waste load instead.
Why does it recommend such a big tank for goldfish? Goldfish are heavy waste producers that reach 8 to 12 inches. Three of them genuinely need 40-plus gallons.
Why does it warn about my betta? Male bettas fight each other and nip long-finned tankmates. The betta care guide covers safe tankmates.