Can you post a picture of the snails? It sounds vaguely like a snail I have in my cube tank, in which I've been unable to keep any kind of coral in the last 6-8 months because they just disappear. However I thought it was the crabs that are in there (I have like 8 or 9 hitchhiker crabs, they're too fast to be caught .. )
Anyhow, crabs and some cowries aren't necessarily innocent either. You might have crabs in that tank and not know it, it took me 6 months after buying the rock before I started seeing them come out regularly. The thing that annoys me the most is that I inspected each and every rock closely for crabs in holes before putting it in the tank ... they were obviously very deep inside or maybe they were larval and somehow survived the shipping.
