nope there shell will grow with them, i belive your thinking a hermit crab. to keep the shell healthy maby add soem cuddle bone to your filter or straight to the tank, its supose to keep the shells looking nice(or some type of calcium) i never tryed it till jsut a few weeks ago for my stripped nitate snail. i never did this with my mystry snails and there shells got to looing pritty bad and most likly what killed them. darby is really good with snails from what i have been reading she will prob comment