The short answer
A 40 litre tank is a small nano community — enough for one modest school of small fish plus a little cleanup crew, not a mixed crowd. A good plan is a single group of 6–8 nano fish (chili rasboras, ember tetras, or a small neon school) with a few shrimp or a nerite snail. Keep it to one or two species and stock lightly.
Stock by needs, not a formula
Forget “one inch of fish per gallon.” That rule ignores a fish’s adult size, its waste output (bioload), whether it schools, its temperament and your filtration strength. In a 40 litre tank the water still swings quickly, so a single messy fish or an overcrowded school can crash your parameters fast. Choose fish for how they actually live, then add fewer than you think you can.
Sensible 40 litre stocking ideas
- 8 chili rasboras or ember tetras + a small shrimp colony
- 6–8 neon or green neon tetras kept as the only fish
- A trio of endlers or a few male guppies, lightly stocked
- 1 dwarf gourami or a betta as a centrepiece with peaceful tankmates
Avoid active swimmers that need length (danios, larger barbs) and anything that grows big. A 40 litre tank is too small for a proper corydoras group or most cichlids.
Before you add anything
Whatever you pick, cycle the tank first so the filter can process waste, then add fish in small batches over several weeks. Keep up weekly water changes and read our fuller guide on how many fish you can keep. For a step down in size, see how many fish in a 20 litre tank, and for the tank itself browse the best nano aquariums.