saw these great looking lil crabs once in a lfs and they were kept in a tank with the warter filled to the top so i figured they didn't need acsess to dry areas. then the other day saw same crabs in a different shop in a tank half full with rocks they were sitting on. confused now im guessing the first place had a rong set up? they were them little black crabs with red claws. anyone tell me more about these and how they should be kept. 