The short answer
A 20 litre (roughly 5 gallon) tank is a nano, so think in terms of one small centrepiece or a tiny group of micro-fish — not a crowd. Good options are a single betta, a small colony of dwarf shrimp, or a group of 6 nano fish such as chili rasboras or ember tetras. Add gentle filtration, a heater, and stock lightly.
Forget “one inch of fish per gallon”
That old rule is unreliable and often dangerous. It ignores a fish’s adult size, its waste output (bioload), whether it schools, its temperament and how strong your filtration is. A single fancy goldfish “fits” an inch-per-gallon tank on paper but will foul a nano in days. Stock by the fish’s real needs, not a formula.
Sensible 20 litre stocking ideas
- 1 betta + a few shrimp or a nerite snail
- 6 chili rasboras or ember tetras (tiny, peaceful shoalers)
- A dwarf shrimp colony (cherry shrimp breed readily and are fascinating)
- 1 male guppy or a trio of endlers, lightly stocked
Avoid active or schooling fish that need room (neon tetras, danios, corydoras in numbers) and anything that grows large.
Before you add anything
Whatever you choose, cycle the tank first so the filter can handle the waste — see how to cycle an aquarium. Add fish gradually, keep up weekly water changes, and read our fuller guide on how many fish you can keep. For the tank itself, see the best nano aquariums.