My crabs have killed one of my turbos, and those were the tiny blue leg hermits, about 1/5 the size of the snail. . .power in numbers! I've replaced about half a dozen trochus snails, though. Most of those were when I was out of town for two weeks. If you're good about flipping them over, the crabs can't get to them. Anyway, I like having both the hermits and the snails. The snails eat up all the diatoms and the crabs gobble up any uneaten food and pretty much whatever else they can get their claws on. As for numbers, I think I have about a dozen snails (turbo and trochus) and at least a dozen hermit crabs (mostly blue leg, but a few red as well). I'm having a hard time finding things like conchs at my LFS, but my cucumber and brittle star seem to do a pretty good job cleaning up the sand.