The only thing that comes to mind is calcium. if your water is hard this isn't usually an issue, if it's soft the shells will erode and the snails will die before they reach full size.
Good sources of calcium include:
In the water column;
CaCl calcium chloride i.e. driveway "heat" ice melter
CaCo3 calcium carbonate i.e. crushed coral dolemite, lime, seashells etc. This will raise your Kh level and thus your Ph level
Supplemental;
Cuttlebone tied to a rock, the snails will graze it for the calcium Cuttlebone will be found in the bird section at the pet store.
Crab cuisine. Most prepared foods that are designed for hermit crabs shrimp crayfish and lobsters are very high in calcium. this will help your snails as well and since they eat anything, they go after these foods readily.
Other than calcium, all they seem to need is food, water, and a copper free environment.
dave