A cycled 10 gallon tank comfortably holds one betta, or a school of 6–8 nano fish such as ember tetras, plus a few shrimp or a snail. A 20 gallon long holds roughly twice that, or a small community. It depends on the species, not a fixed number.
Ideas that work
10 gal1 betta + 6 ember tetras + a nerite snail
10 gal8 chili rasboras + 10 cherry shrimp
20 long8 harlequin rasboras + 6 panda corys + 1 honey gourami
20 long6 platies + 6 pygmy corys
Too big for these tanks
| Fish | Needs at least |
|---|---|
| Fancy goldfish | 30 gal |
| Angelfish | 30 gal, tall |
| Bristlenose pleco | 30 gal |
| Clown loach, oscar | 75 gal |
Add fish slowly
Add a few fish at a time, about a week apart, and test ammonia and nitrite in between. The bacteria need time to catch up with each new batch.
Check your exact fishOpen the calculator →Questions
Is the one inch of fish per gallon rule right?
Only roughly. A chunky 4 inch fish makes far more waste than four 1 inch tetras, and some fish need more room to swim. Check each species.
Can a goldfish live in a 10 gallon tank?
No. Fancy goldfish grow to about 8 inches and need 30 gallons or more; comets belong in ponds.
General information, not veterinary advice. Updated November 2026.