The short answer
A sensible guideline is one nerite snail per 20–40 litres of tank. Nerites are efficient algae eaters, so a small number goes a long way, and the real limit isn’t waste or overpopulation — it’s food. Too many nerites in a clean tank simply starve. Because they can’t breed in freshwater, whatever number you add is the number you keep, so you can stock them precisely to your algae load.
Why food is the limiting factor
Nerites live on algae and biofilm. In a tank with plenty of algae you can lean toward the higher end (one per 20 litres); in a sparkling-clean, low-algae tank, fewer is better or they’ll run out of food and slowly waste away. Watch their behaviour: if they’re constantly roaming empty glass, there isn’t enough algae and you either have too many or need to supplement.
Scaling to your tank
- 20–40 L nano: one nerite is plenty.
- 60–90 L: two to three.
- 100 L+: three to five, depending on how much algae grows.
If your tank grows little algae but you love nerites, keep fewer and feed algae wafers a couple of times a week. Better too few and healthy than too many and starving.
Stocking them well
Because nerites don’t reproduce (do nerite snails reproduce), you never have to worry about a population boom — unlike pest snails in will my snails take over my tank. Keep the water moderately hard for strong shells (why are my snails dying), fit a lid so they don’t wander out (do I need a lid for snails), and read the full nerite snail care page.