You are correct in guessing your shell is eroding. Glad you caught it early before it worsened. This is caused by very insufficient calcium. What do you provide as calcium source for your snails? What are your pH, KH and GH? Start increasing your calcium levels. This can be done by placing crushed corals, crushed oyster shells, limestones, aragonite, plaster of Paris pucks, marble chips, eggshells (powder form), calcium tablets (Tums/Caltrate), cuttlefish bones or liquid calcium by Kent. All except plaster of Paris pucks and liquid calcium are made of calcium carbonate. Plaster of Paris pucks are basically calcium sulfate and can increase the hardness levels. Liquid calcium by Kent is calcium chloride and increases only the KH, not the pH. Test your hardness levels and pH so you can determine your options.
If you opt to make plaster of Paris pucks, get the Dap brand. Add pellets or granules in the mix and the snails will simply nibble them or let it dissolve in the water. If your water is slightly acidic, it will take a few weeks before the plaster of Paris pucks dissolve completely unless your water is already too acidic that the plaster of Paris pucks will dissolve rather quickly.
