The short answer for a nerite snail
One nerite snail can live in a 5-gallon tank, and a 10-gallon or larger established aquarium is the more forgiving home. The limiting factor is rarely swimming room, because a nerite spends its life gliding slowly over glass, wood, and stone, rasping away surface growth. The real limit is food: a nerite eats algae and biofilm almost constantly, and a small, new, or aggressively scrubbed tank can be grazed bare surprisingly fast. As a working guide, allow roughly one nerite per 5 gallons of planted, mature tank so the grazing regrows faster than the snail clears it.
Why an established tank beats a bigger new one
Nerites in home aquariums generally do not thrive on leftover fish food or sinking wafers the way some other snails do; many largely ignore prepared foods. Their menu is what grows in the tank itself: soft green algae on glass, film on wood, and growth on rocks and plant leaves. That is why a modest but mature 10-gallon with sunlit glass and seasoned wood outperforms a large sterile setup. It is also why nerites are best added after a tank has cycled and begun showing its first film, not on day one as part of a clean-up crew expected to feed itself from nothing.
Their eggs are a sizing footnote worth knowing: females lay small white capsule-like eggs on hard surfaces that do not hatch in fresh water. They are harmless, purely cosmetic, and another reason to keep numbers modest rather than stocking several snails hoping for babies.
Lids, waterline habits, and tankmates
Fit a snug lid and cover filter gaps. Nerites explore above the waterline, can survive out of water for a while, and are regularly found on the floor or inside a filter housing after escaping through a surprisingly small opening. Check the rim and lid underside during maintenance before assuming one has died. They also appreciate hard surfaces to graze and rest on, and they do poorly where copper-based treatments have been used, so read any fish medication plan with the snail in mind and move it out if a treatment is unavoidable.
Tankmates are easy: nerites suit most peaceful community fish, though loaches and puffer-type fish that specialise in eating snails are an obvious exception. Their thick shell and tight-fitting operculum protect them from casual nipping better than most snails manage.
Signs one nerite has out-eaten its enclosure
A nerite that stays closed up for long stretches, repeatedly parks at the waterline, or leaves every pane mirror-clean is telling you grazing has run out. Reduce how much glass you scrub, leave one pane as a feeding surface, and consider moving the snail to a more mature tank rather than adding a second snail to compete for the same film. On the right surface area, one nerite keeps glass impressively clear while asking for almost nothing in return except food that is already growing. Noting which pane is left unscrubbed and where the snail usually rests in KinsLe Family Sharing keeps that grazing plan visible to everyone who cleans the tank.