The short answer
A 20 gallon tank (about 75 litres) is a proper small community — room for two modest schools plus a cleanup crew, or a centrepiece fish above a shoal. A balanced plan is 10–12 small tetras or rasboras, a group of 6 corydoras, and shrimp or a snail. Keep it to two or three species and stock over several weeks.
Stock by needs, not a formula
The old “inch per gallon” rule fails because it ignores adult size, bioload, schooling needs, temperament and filtration. At 20 gallons you finally have room for a real community, but it is still finite: crowd it and you will fight algae, cloudy water and rising nitrate. Choose species by their genuine needs.
Sensible 20 gallon stocking ideas
- 10–12 neon tetras + 6 bronze corydoras + shrimp
- A honey gourami pair above a harlequin rasbora school
- 8 platies with a peaceful bottom group
- A small community: one tetra school, one rasbora school, a snail crew
Avoid angelfish, larger barbs and cichlids that want more length. Keep every schooling species in groups of at least six. Planting and a little hardscape help as well: they steady water quality and give each group its own space, which reduces squabbling in a busier community.
Before you add anything
Always cycle the tank before stocking, then add fish in batches so the filter keeps up. Maintain weekly water changes and acclimate new fish slowly. This tank is close to 75 litres — see how many fish in a 75 litre tank. For bigger, read how many fish in a 29 gallon tank.