The Red Clawed Crabs that fish sellers have been pushing for the last couple of years can survive in a regular aquarium, they have gills just like fish. The only problem is they like to get out of the water occasionally, as they do in the wild. So if you put them in a fish tank with no where to get out of the water they will escape the tank to do so. A one inch crab can squeeze through a tiny hole, you literally have to duct tape any cracks and crevice's along your hood to keep them in.
Some people claim they are a brackish animal
They will coexist with most cichlids as they cant get ahold of them.