The short answer
A 125 litre tank is a comfortable community — enough for two or three schools across different levels plus a centrepiece or a bottom group. A balanced plan is two shoals of 10–12 small fish, a group of 6–8 corydoras, and a snail or shrimp cleanup crew. It also suits a single centrepiece species, such as a pair of gouramis or angelfish, above peaceful shoals.
Stock by needs, not a formula
The “inch of fish per gallon” rule is unreliable because it ignores adult size, bioload, schooling needs, temperament and filtration. At 125 litres you have real flexibility, but the tank still has limits: a strong filter and steady maintenance matter as much as the number of fish. Plan around each species’ adult size and behaviour.
Sensible 125 litre stocking ideas
- 12 neon tetras + 10 harlequin rasboras + 8 bronze corydoras
- A gourami pair above two peaceful shoals and a snail crew
- A livebearer community of platies, guppies and swordtails, sexed to control fry
- A pair of angelfish with calm mid-water tetras (nothing tiny or nippy)
Keep every schooling species in groups of at least six, and avoid overloading with too many species — three or four done well beats eight done thinly.
Before you add anything
Always cycle the tank, then add fish in batches over several weeks so the filter matures. Keep up weekly water changes and match the filter to the load — see our aquarium filters hub. For a bigger build, see how many fish in a 180 litre tank.