Snails require calcium to construct healthy shells. There are many ways to provide this calcium.
Plaster of Paris pucks
These can be home made of in the form of commercial "vacation feeders"
This may be purely to provide calcium or as part of a general snail health product such as "snail milk".
Tums can be used on plain or flavored forms. It can temporarily cause water to become cloudy, especially if dropped on a turbulent area of the tank such as near an air stone.