What you've experienced with the snails is extremely common. If you diligently research all of your purchases before you make them you will avoid a lot of surprises like that. You can also expect that your crabs will occasionally kill your snails for their meat and for their shells from time to time as well.
The Grazing Snails, Part I - Turbo, Trochus, Astraea, and Kin
http://reefkeeping.com/issues/2004-05/rs/index.php