Snails
Nerites clean algae and cannot breed in fresh water, so the population stays where you put it. Mystery snails will breed, and they lay above the waterline.
Overview
Aquarium snails do a genuine job. They graze algae and biofilm off glass, plants and decor, and they clear uneaten food that would otherwise rot in the substrate.
Which species it is decides everything else. Nerites graze hard and cannot multiply in a freshwater tank, because the larvae need salt water to develop, so a nerite lays eggs that never hatch. Other kinds breed freely in fresh water and go from a cleanup crew to a population problem in a few months.
Temperament
There is not much to say and that is rather the point. Snails move slowly, eat constantly and interact with nothing. A snail that has climbed out of the water is usually reacting to water quality rather than exploring.
They are eaten by loaches, pufferfish and some cichlids, which is worth knowing before adding either to the other. Most community fish ignore them entirely.
Care and grooming
Algae and biofilm, supplemented with blanched vegetables and a sinking wafer once the easy algae has gone. A snail in a spotless new tank has nothing to eat.
Shells need calcium in the water. In soft water a shell thins and pits at the tip, which does not reverse, so hardness is added deliberately in tanks kept soft for other reasons. Copper based medications kill snails as reliably as they kill shrimp.
Living and exercise
Five gallons is enough, with a lid, because snails climb out. They fit into almost any freshwater tank that is not stocked with something that eats them.
One thing does not stay in the tank. Apple snails were introduced to Florida and the Gulf coast through the aquarium trade, they are established there, and USGS work records them eating the eggs of native frogs and toads. An unwanted snail goes to another keeper or is euthanised, and never into a pond, a drain or a waterway.
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