Found this listing on Ebay... the though of a crab appeals, but I would want to know potential size, diet, are they truly freshwater or brackish... etc.
They're Cardiosoma armatum, and they don't care about the water so long as their is some to dip into. But they are terrestrial. They need a vivarium, with only a pool of water to wet their gills in periodically. Read up on land hermit crabs; they're essentially the same, except prettier. Cannot be kept in an aquarium.
Eat anything. Mine loved soft fruit and the odd frozen whitebait.