calcium

Check and make sure that the ingredient is just calcium carbonate. If so, you're good to go. If it has other things like potassium, I would avoid it for the snails.

The amount of calcium you have to add directly to the water may depend on how hard your water already is. I have very soft water, and I have to raise my kh/gh as well as add calcium to the tank daily. I still see too much shell erosion. I have calcium chloride (kent's liquid calcium). I'd advise looking for the calcium carbonate as suggested above, especially if you have soft water. Some people with hard water keep healthy snails without adding calcium at all.

Water hardness and calcium supplements added to the water will effect how much shell erosion you see. **Meanwhile, calcium intake (via high calcium foods) will provide the calcium needed for new shell growth.** <-- This is my assumption, though I don't know how much calcium snails might take in directly from the water to use for new growth. I suppose some calcium from the water may be soaked up by whatever the snails feed on.

www.applesnail.net is a wonderful resource with lots of great information and pictures.
that being said, its important to offer your snails both an appropriate environment to help their shell stay healthy and food to make it grow healthy. There are a couple of products out there that can help if your water is too soft (wondershells, vacation feeder blocks, cuttlebone). I feed my snails a variety of foods, many of which I make myself. I feed hikari crab cuisine, sinking wafers, and a mixture of calcium rich snail jellos and biscuits as well as veggies.