The group of crabs known as fiddler crabs actually encompasses over 90 species and subspecies of the genus Uca that come from a variety of locations.
The fiddler crabs found in pet stores are most likely semi-terrestrial brackish crabs, which means they need some salt in their water as well as access to air and dry land. Unfortunately, many pet stores keep fiddlers in a freshwater aquatic set up, and recommend the same to new owners. They may do fine in a fresh water and fully aquatic set up for weeks, but will eventually become weak and die