Fiddler crabs, with that one huge claw, are sold as freshwater aquarium pets, but the instructions usually given won't keep them alive for long.
Fiddler crabs are interesting tiny creatures that are found in brackish waters, either in mud or sand. They don't ever grow much over two inches from the end of one leg to the end of the opposite leg on the other side of its shell. The male has one claw that seems far too huge for his body. He waves it about to ward off enemies and to attract females. In the aquarium they are happiest in pairs, as sometimes one female will kill extra crabs in her tank. This is less likely to happen in very large tanks. It is recommended to house them in at least a ten gallon tank. They do not thrive as single pets.
Fiddler crabs are often sold in pet shops as freshwater inhabitants. While in the stores they may be kept entirely underwater, and their water will contain no salt. Unfortunately for the crabs, they cannot live this way for long, and in under a month you can expect a fiddler kept in such conditions to die.
Fiddler crabs must have access to dry land, as well as slightly salty water. They spend part of their time under the water, and part of their time out on the sand. The simplest solution is to line the aquarium with three to six inches of aquarium sand. Aquarium gravel will work, too, but they cannot tunnel through it as easily.
Pile it 6" to 8" high in one end, but not so high that the crabs can reach the top of the tank, because they will escape. Make the other end no deeper than an inch. Create a slope on which they can climb. Place a cup or bowl in the shallow end and slowly fill the tank with water until only about 1" of the high end of the sand beach is exposed.
Add dechloraminator to the water if you use tap water. Next add about one tablespoon of kosher salt or two tablespoons of rock salt to the tank. Filtration is optional.
You will need to top the tank off with new water every week or so to replace evaporated water, but do not add more salt. The salt will not evaporate at the same rate as the water, and your crabs will still have enough.
Crabs are not affectionate pets, but they are quite interesting to watch. The little male waves his giant claw about in a semaphore like fashion when you approach, or when he wants to impress the lady crab. The crabs will dig tunnels in the sand and they may scuttle into them to hide if you're not very stealthy.
Fiddler crabs eat microscopic critters that show up in the sand as well as algae. The female crams food into her mouth one tiny claw after the other, but the male cannot eat with his large claw and must feed himself solely with the little one. The crabs sift the sand through their systems and get nourishment from the microscopic particles and bacteria in it.
Periodically the crabs will shed their shells. As they ease themselves out of their old shell that has grown too small, you can see a whole new set of eight legs and two claws curled up inside. If they lose a leg or claw it will grow back inside the shell and be ready the next time they molt. If the male loses his large claw, when he molts the big claw will be on the opposite side from where it was. They must be careful for a few days until the new shell hardens.
Some aquarists remove the shells from the water, but this is not advised. The crabs will consume the old shell over several days and it will provide them with the calcium they need to develop a hard new shell. If the old shell remains in the water for more than a week, it may be removed, but it is unlikely that this will be a problem.