Hello, I kinda new to the hobby, but everytime I buy a new snail my blue legged crab kills them?? He has taken one of the shells but why kill all 3 of them? The crab keeps changing from his original shell to the snail shell. My snails are the turbo snails. I have 2 red legged crabs that stay away from the blue legged crab. AI am afraid I will not have a good clean up crew? Any advice would be great!!!