fiddler crabs are brackish and semiterrastrial,so no good for most aquariums.you can get several creatures for a freshwater aquarium that are fully aquatic.shrimp,crayfish,frogs,and 2 varietys of crabs i know of,panther crabs and thai micro crabs.the frogs are african dwarf frogs.crayfish may eat your fish,but it depends on there personality and how well fed they are.