Fish tank stocking calculator
Enter your tank and the fish you want. You'll see straight away whether they can live together, whether the tank is overstocked, and what to change.
1. Your tank
More options (tank length, filter, plants)
2. Your fish
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Result
Add your tank and fish
100% is a ceiling, not a target. Under 85% leaves room for growth and mistakes.
Fish you could add
These fit the space left and get along with everything you've chosen.
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How many fish can you keep? Add up each fish's adult size and waste, compare it with what your tank and filter can process, then check that every fish shares the same water and won't eat or bully the others. This calculator does all of that for you.
How the calculator works
Each species in our fish database has researched values for adult size, waste output, temperature, pH, hardness, minimum tank size, school size and behavior. The calculator uses them in five checks:
- Waste and space. Bigger fish make much more waste than small ones, so we weight each fish by its adult size, not its size in the shop. Goldfish, plecos and large cichlids get an extra multiplier because they are especially messy. Space is measured separately: a stronger filter helps with waste, but it never adds swimming room.
- Water. We find the temperature, pH and hardness ranges that suit every fish at once. If there isn't one, they can't share a tank.
- Compatibility. Every pair is checked for predation (who fits in whose mouth), fin nipping, aggression, territory, and shrimp or snail safety.
- Tank size. Some fish need a minimum volume or tank length to swim properly, no matter how few you keep.
- Groups. Schooling fish kept in small numbers get stressed and sick, so we warn when a group is too small.
Read the full details in how our stocking calculator works.
Why not "one inch of fish per gallon"?
The old rule says a 20 gallon tank holds 20 inches of fish. But a single 10 inch fish weighs and pollutes far more than ten 1 inch fish, and the rule ignores filtration, behavior and water needs. Use it only as a very rough starting point for small, slim fish.
Tips for a healthy, well-stocked tank
- Cycle the tank before adding fish. See our guides on the nitrogen cycle.
- Add fish in small batches over several weeks so the filter bacteria can keep up.
- Stay under about 85% if you're a beginner. It gives you room for mistakes.
- Change 25–30% of the water every week and test for ammonia, nitrite and nitrate.
Frequently asked questions
How many fish can I put in my tank?
It depends on the adult size of each fish, how much waste it makes, your filter and how often you change water, not just gallons. As a rough guide, a well-filtered 20 gallon tank can hold about 30 neon tetras on their own, or one fancy goldfish. Enter your fish above for a real answer.
Is the one inch of fish per gallon rule accurate?
No. It treats a 10 inch fish like ten 1 inch fish, but a large fish makes far more waste than several small ones. It also ignores filtration, tank shape and behavior. Our calculator weights fish by adult size and waste instead.
What do the waste and space meters mean?
Waste compares the estimated waste of your fish at adult size with what your tank and filter can handle. Space compares their adult body size with the water volume. The higher of the two is your stocking level. Under 85% is comfortable, 85–100% needs strong filtration and weekly water changes, and over 100% is overstocked.
Does a bigger filter let me keep more fish?
Only on the waste meter. Strong filtration (8 or more times the tank volume per hour) raises waste capacity by 10–15% in our calculator, and dense live plants add 10%. Neither adds swimming room, so the space meter stays the same, and neither makes incompatible fish safe together.
Are my plans saved?
Your plan lives in the page link, so you can bookmark or share it. You can also save it to My tanks, which keeps it in this browser only. We never upload your data.