Unless you have really soft water, regular water changes should replenish the water with any calcium that is being used (and there really isn't a lot of usage of calcium in FW that I know of...). I've also read you can feed a snail a piece of blanched spinach to supplement calcium. Take the uneaten portion out of the water after a few hours.
Snails - inverts in general really - are more sensitive to water quality issues or contaminants. Something like the metal weight used on a bunch of plants, a hunk of rust or a coin that fell in, a metal screw used on something in the tank, or using any of a variety of medications could affect your snail but not the fish in the tank. Or even a REALLY high nitrate level.
He didn't get caught in a filter or something? I've got a nerite that made his way into my filter and got the edge of his shell all banged up like that.