If you want to keep their shells hard, a piece of cuttlebone will help deliver calcium.
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Boiled and cleaned egg shell, ground or crushed works. I had a nerite that had holes in the shell, did the egg shell thing (I blended it up with veggie debris from making omelets one day and a small baby food, froze, then fed occasionally) and the holes filled in. It took a few months, but it worked.
Apparently, so does adding sea shells, because they degrade over time, releasing calcium. I haven't noticed any new holes on my snails, and I apparently have something in the neighborhood of 300 RCS now, so there's a pretty good need for calcium in there.