not to sure what they will do, but i cant see them hurting any fish. when i took my son camping in Wales UK we used to use them to catch crabs. (no innuendos) please. ;op
They are harmless and act much the same as snails in their habits, but slower. I have them in several of my tanks and the type I have do not get much over 2mm in size.
they're not bad or harmful, but they are one of those creatures (like snails, worms, etc.) whose population depends upon the amount of waste/uneaten food in the tank. so keep that in mind.
Not harmful, there are seveal species that I've seen in the market.
There is a Floridian species that is clear like Bill stated '2mm'
There are some tropical species in the Nerite family from the genus's Septaria and Nepterion which are larger but from past experience more sensitive to water condtions.