My snail tank has a crushed coral substrate...Looks great and their shells are PERFECT, even my larger specimines are flawless...Snails (particularly if you have many) suck calcium right out of the water at a very high rate...With the crushed coral they will never deplete it... Exposed areas wher the shell has erroded will cause the flesh to calcify in those spots, but it will never really grow back..Like Dave said, sont be suprised if they dont make it if the dammage is bad.