Crawfish?

cray fish like to hide. They will dislodge gravel to get under rocks or what ever is setting on the floor of the tank. They are out more at night to forage. And if they do not work out don't let them go in your local waters. They will make excellent food for cichlids with a little manipulation. Or give them to a friend.
 
A fidler crab does not know the difference between standing on the tube of a filter or a piece of dirt.
 
Fiddler crabs are mainly terrestrial dwellers. In their natural habitats, they spend most of their time on land, and can dig burrows several feet deep. That's very different than the tube of a filter. Giving them that little space is much like flooding a town and giving people nothing but lightposts to stay out of the water.
 
A fidler crab does not know the difference between standing on the tube of a filter or a piece of dirt.

Fiddler crabs do indeed know the difference. They live in burrows that they dig themselves, above the water line. They are also extremely territorial, but live in communal groups. If you would study their natural habitat and behaviors, you could make a much better environment for fiddler crabs.

FYI, they also need access to fresh water(drinking/bathing) and brackish water (foraging and skulking ;)).

http://en.wikipedia.org/wiki/Fiddler_crab Read up!
 
Fiddler crabs don't need freshwater, as I doubt they'd really come upon it in the wild. It's probably just a misconception created when hermit crab keepers started keeping fiddlers.
 
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