How many fish in a 20 gallon tank?
Short answer: roughly 20 small, slim fish (like neon tetras or rasboras) in a well-filtered 20 gallon tank — far fewer if they grow larger. Good options: Neon tetra, Harlequin rasbora.
Tested stocking ideas
Classic tetra community
10 × Neon tetra, 6 × Panda cory, 1 × Dwarf gourami
Rasbora and honey gourami
8 × Harlequin rasbora, 1 × Honey gourami, 6 × Otocinclus, 3 × Amano shrimp
Species that fit a 20 gallon tank
| Species | How many (alone) | Note |
|---|---|---|
| Betta fish | 1 | one male |
| Bristlenose pleco | up to 2 | |
| Cherry shrimp | up to 99 | groups of 10+ |
| Dwarf gourami | 1 | one male |
| Fancy goldfish | 1 | |
| Guppy | up to 16 | groups of 3+ |
| Harlequin rasbora | up to 18 | groups of 6+ |
| Molly | up to 5 | groups of 3+ |
| Neon tetra | up to 36 | groups of 6+ |
| Platy | up to 12 | groups of 3+ |
| Amano shrimp | up to 85 | groups of 3+ |
| Black skirt tetra | up to 12 | groups of 6+ |
| Bronze cory | up to 9 | groups of 6+ |
| Cardinal tetra | up to 36 | groups of 6+ |
| Celestial pearl danio | up to 68 | groups of 6+ |
| Cherry barb | up to 18 | groups of 6+ |
| Chili rasbora | up to 95 | groups of 8+ |
| Ember tetra | up to 83 | groups of 8+ |
| Endler's livebearer | up to 45 | groups of 3+ |
| German blue ram | up to 17 | |
| Glowlight tetra | up to 29 | groups of 6+ |
| Honey gourami | up to 14 | |
| Kuhli loach | up to 10 | groups of 5+ |
| Mystery snail | up to 42 | |
| Nerite snail | up to 99 | |
| Otocinclus | up to 33 | groups of 6+ |
| Panda cory | up to 17 | groups of 6+ |
| Pea puffer | up to 45 | |
| Rummy nose tetra | up to 18 | groups of 8+ |
| Serpae tetra | up to 22 | groups of 6+ |
| White cloud mountain minnow | up to 29 | groups of 6+ |
| Zebra danio | up to 18 | groups of 6+ |
| Black neon tetra | up to 29 | groups of 6+ |
| Bloodfin tetra | up to 16 | groups of 6+ |
| Cockatoo dwarf cichlid | up to 8 | |
| Dwarf neon rainbowfish | up to 8 | groups of 6+ |
| Forktail blue-eye | up to 16 | groups of 6+ |
| Glass catfish | up to 14 | groups of 6+ |
| Lemon tetra | up to 18 | groups of 6+ |
| Paradise fish | 1 | one male |
| Peppered cory | up to 10 | groups of 6+ |
| Pygmy cory | up to 52 | groups of 8+ |
| Sparkling gourami | up to 23 | |
| Sterbai cory | up to 10 | groups of 6+ |
| X-ray tetra | up to 24 | groups of 6+ |
How we calculate it
We count each fish in "units": one slim 5 cm (2 inch) fish = 1 unit, and a well-filtered tank holds about 1 unit per US gallon. Units scale with (adult length ÷ 5 cm)1.5 × a body-mass factor, so a 15 cm angelfish counts as about 8 units, not 3. Species are excluded if their recommended minimum tank is larger than 20 gallons.
Other sizes: 5 gallon · 10 gallon · 29 gallon · 40 gallon · 55 gallon · 75 gallon
FAQ
How many fish can I put in a 20 gallon tank?
It depends on the species. As a conservative guide, a 20 gallon tank holds about 20 small (2 inch) slim fish, far fewer large or heavy-bodied fish. The table above lists the maximum for each suitable species kept on its own.
Is the 1 inch per gallon rule accurate?
Only for small, slim fish. Waste output grows much faster than length, so a 6 inch fish produces far more waste than three 2 inch fish. We scale by body size and body shape instead.