Are all the snails that are having problems from a recent purchase? What species of snails are they? When I started my first reef tank, I purchased a bunch of Nassarius snails on ebay. They were way cheaper than anyone elses...lol. I soon found out why. They were Nassarius snails alright, but were from temperate waters instead of tropical. They didn't last long at all. Of course if they aren't all from a recent purchase, I don't know what else to say. Have you had a chance to check for stay voltage yet?