I agree, what is the temp? How old is the snail? Has it reached sexual maturity yet? What do you feed? Your pH is ideal, do you have any idea what the hardness of your water is? Its likely adding crushed coral will not help in this case, as it buffers the hardness in order to raise the pH to prevent shell erosion. The new growtth is kept healthy by a calcium rich diet. You can achieve this through the home made foods, or fresh or frozen veggies, or even feeding vacation feeders or wonder shells (made by wardley, like a vacation feeder). Can you get a picture of the damage?