I have never, kept them, but I'd say pretty hard as I have only ever seen them in public aquaria but most of this information is from memory from a book.
You need to have a gently shelving 'beach' of sand (which is easiest to have lengthways in teh aquarium to make it shallower) where they can climb onto, and floating some driftwood which they can climb onto is also recommended. IN public aquariums they often have little wave machines in their displays.
6 inches of water is enough but the air must be kept humid so a good cover is required.
They are carnivores and can become quite tame but they have sharp teeth lol.
THey grow to about 6 inches, and its best to just keep a few.
I dont know whether they are relatively hardy as long as the set-up is fine, it seems to me that giving them the right aquarium is the most important thing.
Unless you have had a fair bit of experience, I wouldn't suggest these because they are probably quite expensive.