You'll need a larger tank for them. The minimum tank size is 5 gallons. They require brackish water and they need a terrestrial setup. This means that they need access to land and water. They prefer sand in their habitat, since sand is easier for them to dig and burrow in. Sometimes the petstores may keep them underwater, but they need access to dry land. They may be able to live underwater for a few weeks maybe even a couple of months, but they will have a shorter life. Whatever they might tell you at the store purchase marine salt instead of aquarium salt. They need a species only tank as they may eat any other tank mates not of their species. Below is a list of items you'll need for their setup.
tank (5g minimum)
heater (keep the water around 75-82F)
filter (you can purchase a canister filter)
sand (some have said pool filter sand or play sand is ok)
marine salt (not aquarium salt)
hydrometer (used to measure salinity of water)
thermometer
food (crab food with calcium helps them when molting)
water dechlorinator
secure lid (they have been know to wonder out of their homes)
decorations (they are optional, but make the tank look good)
When purchasing your fiddler crabs look for ones that are active. Dont house to many males in one setup or they will fight till only one is left. Males have a one large claw and a small claw. Females have two small claws. They like to climb so make sure that your lid on your aquarium is secure. You should setup the tank before you purchase the crabs. And I've been told that you have to wait at least 3-4 days after mixing the salt and adding it to the tank before you can get your crabs. The crabs will molt every now and then, but dont remove the molt as they will eat it for calcium. If it is not eaten after a few days you then can remove it. You can feed them ghost shrimp as they have calcium, but some have told me that their crabs leave them alone. You'll have to do water changes also.
I'm setting up my 10 gallon tank for fiddlers and I've got only until Friday or Saturday until I get mine. You can even look into buying a 10 gallon, because I went to Petsmart and got mine for $10. And with the 10 gallon you can get more crabs. Im planning on getting 3-4 fiddlers in my 10 gallon, but thats all females.