Fish Tank Stocking Calculator
How many fish can your aquarium safely hold? Enter your details below โ the calculator adjusts for species type and filtration strength.
Calculate your stocking level
Result shows the maximum safe stocking level. Stocking slightly below the maximum is always safer.
How the stocking calculator works
The calculator starts from the well-known inch-per-gallon rule: one inch of adult fish length per gallon of water. This rule works as a rough baseline for slim-bodied tropical community fish under about 3 inches long.
But not all fish are equal, so the calculator adjusts by species:
| Fish type | Rule used | Why |
|---|---|---|
| Small tropical community | 1 inch per gallon | Slim bodies, low waste |
| Fancy goldfish | 20 gal first fish + 10 gal each extra | Heavy waste producers |
| Common / comet goldfish | 40 gal first fish + 30 gal each extra | Grow to 12+ inches; pond fish really |
| African cichlids | 1 inch per 2 gallons | Territorial and messy |
| Betta | 5 gallons per fish | Minimum humane tank size |
Filtration then scales the result: excellent filtration (an oversized canister filter, for example) supports about 15% more stocking; a basic or brand-new filter means you should stock about 20% lighter until the nitrogen cycle is established.
Common stocking mistakes to avoid
- Counting juvenile size. That cute 1-inch goldfish will reach 6โ8 inches. Always use adult size.
- Ignoring behavior. Territorial fish need footprint and hiding spots, not just water volume.
- Stocking all at once. Add fish gradually over weeks so beneficial bacteria can catch up.
- Forgetting the nitrogen cycle. A new tank needs 4โ6 weeks to cycle before full stocking.
Frequently asked questions
How many fish per gallon of water?
The classic rule is one inch of adult fish per gallon for small tropical community fish. A 20-gallon tank therefore supports about 20 inches of fish โ for example, ten 2-inch tetras. Heavier-bodied fish like goldfish need far more space each.
How many fish can I put in a 10 gallon tank?
About 10 inches of small tropical fish: 6โ8 neon tetras, 5โ6 guppies, or a single betta with a few peaceful tank mates. A 10-gallon tank is not suitable for goldfish long-term.
How many fish for a 20 gallon tank?
Roughly 20 inches of tropical community fish โ for instance, 10โ12 tetras plus a small school of corydoras. Or 1โ2 fancy goldfish at most.
Does filtration affect stocking?
Yes. Strong filtration with plenty of biological media processes more waste, allowing roughly 10โ15% heavier stocking. With a weak or uncycled filter, stock lighter.
Is the inch-per-gallon rule accurate?
It's a starting guideline. It works reasonably for slim tropical fish under 3 inches but breaks down for large, messy, or territorial species โ which is why this calculator adjusts by fish type.