Last weekend I picked up a yellow mystery, a purple mystery (sold as black but looks purple) and a nerite snail. I've since been reading about snail care, as I really don't know much. It looks like some people add calcium into their tanks for their shells. My KH is around 6 and my GH is around 19, so I have hard water. Would I need to add calcium if I have hard water already? My pH tends to range between 7.4-8.0, so I don't really want to raise it. The purple snail doesn't have the best shell and it has a hairline crack in it and it looks sort of dry, so I'm a bit concerned about calcium.