Bad mystery snail shell. Need help!?

I have 4 mystery snails. One of them has a problem with his shell. It looks like the top layers are flaking. I have two zebra snails as well. They don’t have any problems. Why does he have a bad shell and the others are fine? I read that adding calcium to the tank can help. I know of calcium additives for reef tanks. Can I use that in my freshwater tank? How much do I use? Will it raise the ph? I have seen pictures of snails that have been painted with something clear to help harden the shell. What is that? Is that something I should do?