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Aquarium Stocking Calculator
Pick your tank size, add the fish you're considering, and get a stocking percentage based on adult size and bioload — plus group-size, temperature and compatibility warnings.
Open the calculator →How to Cycle a Tank
More new fish die from uncycled tanks than from any other cause. Our step-by-step fishless cycling guide explains what to do, in what order, and how to know you're done.
Read the guide →The Inch-Per-Gallon Myth
The famous "1 inch of fish per gallon" rule says a goldfish and a school of tetras are the same. They aren't. Here's why the rule fails — and what to use instead.
Read why →Why our numbers are different
Most stocking advice online is a guess repeated from site to site. We do three things differently:
- Adult size, always. Fish are sold as babies. Every number here uses the size the fish will actually reach.
- Bioload, not just length. A chunky, heavy-waste fish counts for more than a slim tetra of the same length — because your filter experiences them differently.
- Comfort first. Our "100%" is a comfortable ceiling, not a survival limit. A tank at 70–90% forgives a missed water change; a tank at 130% doesn't.
Ready-made stocking plans
10-Gallon Stocking Plans
Five complete, tested combinations for the most popular first tank — from a betta with a snail to a chili rasbora nano community.
See the plans →20-Gallon Stocking Plans
Four balanced community plans for a 20-gallon, each with its calculator-verified stocking level and the temperature to set. 29- and 55-gallon plans are also available →
See the plans →Best Beginner Fish
Eight hardy, peaceful species that forgive beginner mistakes — with honest notes on the popular fish we'd steer you away from.
Meet the fish →More calculators
Heater Size Calculator
Too small and your fish chill overnight; too big and a fault can cook them. Work out the right wattage for your tank and room.
Calculate wattage →Water Change Calculator
Turn "change 30% weekly" into exact gallons, litres and buckets — for any tank size and any percentage.
Calculate a water change →Tank Volume Calculator
Second-hand tank with no label? Work out its gallons and litres from the measurements — and why footprint matters as much as volume.
Measure your tank →