1. Your tank
2. Your fish
3. Results
Understocked
0% of safe capacity
Warnings
How the stocking rules work
The inch-per-gallon rule
The classic beginner guideline: one inch of adult fish per gallon of water. A 20 gallon tank can hold roughly 20 inches of fish. Saltwater is less forgiving, so this calculator halves it to one inch per two gallons for marine tanks. Use adult sizes, not the size at the store.
The surface-area rule
Fish consume oxygen at the water surface, so surface area matters as much as volume. The rule: one inch of fish per 12 square inches of surface for freshwater, and one inch per 48 square inches for saltwater. Tall narrow tanks stock lighter than wide shallow ones of the same volume.
Both rules are starting points for beginner community fish. Heavy waste producers (goldfish, plecos), sensitive species (discus), and aggressive fish all need more room than the rules suggest. This tool shows the more conservative of the two rules and is general guidance only. Always research each species before buying.
Frequently asked questions
How many fish can I put in a 10 gallon tank?
About 10 inches of adult freshwater fish, for example 6 neon tetras and 4 cherry barbs, or a single betta with 6 small schooling fish. Add the species to the calculator above with a 10 gallon tank to see the exact stocking level.
Is the inch-per-gallon rule accurate?
It is a rough starting point, not a law. It underestimates big-bodied or messy fish like goldfish and overestimates for tiny active fish. Pair it with the surface-area rule and temperament checks, which this calculator does automatically.
Can I mix saltwater and freshwater fish?
No. They need completely different water chemistry and cannot share a tank. Use the Freshwater / Saltwater toggle above and the calculator will warn you if you mix species across water types.
Do schooling fish need a minimum group size?
Yes. Tetras, rasboras, danios, barbs, corydoras, and chromis stress when kept in tiny numbers and should be kept in groups of 6 or more. The calculator flags any schooling species below its minimum group size.
Can I keep a betta with other fish?
Male bettas are aggressive toward other male bettas and nippy toward flashy long-finned fish, but many do fine in a community tank with peaceful small tankmates in 10 gallons or more. Never keep two male bettas together.
How do I calculate tank gallons from dimensions?
Multiply length x width x height in inches and divide by 231. The calculator does this automatically when you enter dimensions.
Disclaimer
Species sizes and care notes are general guidance compiled from widely published aquarium references. Individual fish vary, and care requirements depend on filtration, water parameters, tankmates, and experience level. This calculator is a starting point, not a substitute for researching each species before purchase.