The short answer
A 100 litre tank is a proper community — enough water for two or three peaceful schools on different levels plus a cleanup crew. A balanced plan might be an upper-level school of 10–12 tetras, a mid-level group such as a gourami pair, and a bottom group of 6+ corydoras. Bigger and more forgiving, but still stocked gradually.
Stock by needs, not a formula
The “inch of fish per gallon” rule fails at every size. Instead weigh each fish’s adult size, bioload, schooling needs, temperament and your filtration capacity. A 100 litre tank gives you more room and a bigger safety margin, so you can keep active swimmers and full schools that would be cramped in a nano — but overstocking still causes algae, poor water and aggression.
Sensible 100 litre stocking ideas
- 12 rummynose tetras + a honey or pearl gourami pair + 8 corydoras
- A small rainbowfish school with a peaceful bottom group
- A modest livebearer community (platies, mollies) with plenty of plants
- A single centrepiece such as a dwarf cichlid pair with a nano dither school
Avoid overcrowding levels or mixing aggressive and timid species. Strong, well-maintained filtration matters more here than a precise fish count.
Before you add anything
Cycle the tank first, then add each school in batches over several weeks so the filter can keep up. Keep up weekly water changes and read our fuller guide on how many fish you can keep. For a smaller build, see how many fish in a 60 litre tank, and browse the aquariums hub for the tank itself.