Sorry, I haven't kept fiddler crabs myself, so I'm not expert, but that does not really seem like strange behavior. Most fiddler crabs prefer to be on land part of the time and in the water part of the time, so optimally you'd want to have something for them to sit on out of the water (such as rocks, a floating platform, etc.)
Every time I've been to the beach I've seen lots of them runining around near the motels, so it seems like they're sometimes out of water a large portion of the time.
Also, is this a fresh water aquarium? Fiddler crabs are either brackish or fully saltwater crustaceans, so they won't live out their life in freshwater and will eventually become weak and die.
Here's a website that has a bit of info and some links: