Look online for Queen Conch, they are only avaible through capitive breeders because they are endangered. They can get pretty big, up to a foot, but isn't very likely at all. The eat algea (red slime), and stir sand. I've found online for like $7 or so, and are recommended 1 per 25. I've heard pufferlover mention Queen Conch in an aggressive tank, I haven't heard much from him as of late. Here's a pic of a queen conch shell that I found right off the dock of the house I stayed at when I was in Belize, the bottle cap there is just for comparison.
But with aggressive fish, you can never be sure. I suggest getting a few cheap hermits, snails, and a queen conch or two, but don't go out and spend $100+ on a prepackaged deal.
-Mark