i had to sprinkle aragonite around my tank for these guys:
lack of carbonates in the water and low PH is what causes them to erode. to help them develop new shell, they need access to food with calcium in it. nerites that have access to hair algae and BBA usually get enough calcium from their food to produce strong thick shells. they produce a coating on their shell that is resistant to corrosion, so they usually do better than the ramshorn snail i posted a pic of, but once spots show up, you really need to do something about it. otherwise, the corroded spots just keep growing and growing.
anything you can do to add calcium into the water will slow down the corrosion, and anything you can do to add calcium to their diets will help them produce thicker shells. you will never see the spots that are already there go away, but you can slow down the progression.