This is from a webpage
"Keeping Fiddler Crabs
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. If possible, find a store that keeps them in brackish water, or wait for a new shipment so their time spent in fresh water is minimal. Look for crabs that are active and have all their legs and claws.
Providing Brackish Water
Because most of these crabs are naturally from brackish waters, most experts recommend putting salt (get aquarium salt from the pet store; never use table salt) in the water. The ideal amount of salt to add is controversial, but it is probably best to get a hydrometer and add enough salt to the water to attain a specific gravity of around 1.005 - 1.010. Hydrometers are not all that expensive and can be found at pet stores and also home brewing shops. Alternatively, your package of aquarium salt may have instructions for producing brackish water conditions. It is okay to vary the salt concentration/specific gravity slightly as these crabs would naturally experience some variations in salinity.
Fiddler Crabs Need Land Too
Some provision should be made to provide the crabs with access to land/air. Owners that do not do this often find their crabs crawling up the filter intake and into or on top the filter in an attempt to find land. Providing a sloped bottom to the tank with part of the gravel or sand out of the water works well. Alternatively, use a partly filled tank with large rocks on which the crabs can climb out of the water."