Crabs are dirty, and they have Houdini-like escape skills. They can swim (it is hilarious to watch, they kind of jump and wiggle their legs really fast), and if you have a back-mounted filter, they will climb it, even if other means of getting into open air is provided. Make sure your tank is low enough that they cannot climb out, because if they can, they will... I'd recommend the water level be 2.5 inches below the lip of your tank at a minimum. As I said, they have an uncanny ability to escape from the aquarium. If I recall they are also brackish water animals, so you may have to take that into consideration as well.