Many of the little hitchhiker snails breed asexually so you can end up with an infestation. Unless you have fish that eat them it's best to remove them. I'm keeping all mine as I will have a small puffer that eats them, otherwise best to get them out.
Same happened to me, but found removing them didn't work... I've accepted them being there but now have several assassin snails who go around keeping the numbers in check.