I have had a bunch of sand sifting starfish for over 2 years now in my reef. They stick to consuming microfauna in the sand and are not looking for snails or crabs to eat. As it was stated though they can deplete their food supply so its best to ensure the sand bed can support them with plenty of micro fauna. I don't clean my sand bed as I let the starfish and the cucumbers do that work. Depending on the type of snails you get some do better than others. I recommend black foot trochus snails as they are hardy, breed in captivity, can right themselves easily, require less food compared to mexican turbos. Hermit crabs always slowly die out either because they can't get new shells or fight with each other over shells/food.