Painting them with nail polish really is a bad idea. The chemicals in the polish are really harmful to any shelled animal. THe suggested fix for eroded shells is egg shell patch stuck on with crazy glue. You can crush the eggshell into powder and mix it with the glue and paint if on too. Just be sure not to get any on the snails soft parts. Any natural calcium source is fine for the snails. You can make your own calcium blocks with drywall patch. Just make sure it doesn't have any mold inhibitor in it. Applesnail.net has lots of recopies for supplementing their diet. If you have a PH below 7 you need to supplement the calcium to keep them from having damaged shells. You can use human calcium supplements, eggshells (no need to boil just rinse and drop in), seashells, antacid tablets like Tums, cuttlefish bone, or any green plants high in calcium like dandelions, spinach, kale, anything they will eat.