The short answer
A 100 litre tank comfortably holds a group of 8–10 bronze corydoras (or another single species) with plenty of room for a mid-water school above them. Corydoras are social bottom-dwellers, so keep at least six — larger groups are more active, more confident and far more fun to watch. There is easily space here for a full community built around them.
Why group size and substrate matter
The old “inch of fish per gallon” rule ignores what corydoras actually need: numbers, the right substrate and a moderate bioload. They are shoaling fish, so six is the minimum and eight to ten is better. They also sift the bottom constantly, so a soft sand or smooth rounded gravel substrate protects their delicate barbels. Volume alone does not decide the count — behaviour and floor space do.
Stocking corydoras well in 100 litres
- 8–10 bronze or panda corydoras below a neon or rasbora school
- A dwarf-corydoras group of 10+ pygmy corydoras if you prefer nano tankmates
- Corydoras + a peaceful mid-water shoal + a shrimp or snail cleanup crew
- Give them open sandy areas and a few shaded hides
Avoid sharp substrate, aggressive tankmates and keeping them in ones or twos — a lone cory is a stressed cory.
Before you add anything
Always cycle the tank before adding your group, then introduce them in one or two batches and acclimate slowly. Keep up weekly water changes. For the full stocking picture, see how many fish in a 100 litre tank and read how many fish you can keep.