You get a lot of different opinions about the hermit crabs in particular. I am somewhat anti-crab. My problem with them has been with their killing my snails, especially the Nassarius. I have not heard of problems with them killing each other, unless there were not shells available. I think that is the cause of many of the snail fatalities as well. When a crab outgrows its shell, it has little choice but to do whatever it can to obtain a larger one. However, that doesn't mean they have to do it in my tanks. Other people have had good luck with hermits, including the scarlets. Perhaps it is the size and quantity of the crabs and the snails in the tanks that determines whether the crab has any interest in those shells?