The aquarium setup should be mostly sand, with a small pool of shallow water at one end. These crabs are mostly terrestrial and need to be able to leave the water. They also like to burrow in sand or mud. They will appreciate a little salt in the water, but it should be on the low end of brackish. For a short stay full fresh water is fine, especially if you're not sure how much salt to add.
They are omnivores and will eat a wide range of kitchen leftovers. Small bits of raw meat are probably preferred, but they should take vegetables and even bread too. Most pet stores will have a pellet food formulated for hermit crabs that should be a good match for their long term nutritional requirements.
I can't recommend releasing it at this point because of the possibility of introducing diseases back to the wild population after its extended stay in contact with humans and especially the fact that you don't really know where it came from. But, since it's probably going back anyway, are you sure it came froom the beach? Fiddler crabs are brackish water creatures and are more likely to be found on the shore of a river or creek connected to the ocean. Have you been anywhere like that?