Crabs Attacking Snails

I have a tank that contained a mixture of snails (turbos, margarita, etc.) There were probably five varieties. I also have two or three red legged crabs and two or three blue legged crabs. The problem is that every time I put some snails in the tank (65 gallon reef) they are attacked by the crabs. Anyone have ideas about whether this is just a bad mixture or if something else is going on? I see that many cleaner-packs have a combination of the two, but I'm not sure of anything beyond that. Thanks in advance.