the snails that people talk about taking over your tank are usually the little tiny pond snails that come in with live plants and a lot of the time when there netting the fish for you, sometimes you get some little snails, those will multiply like mad. Other snails are fine like black mystery snails, apple snails, ramshorn snails, they wont take over your aquarium, and if the try, there much bigger and can be much easier to control