Cool!
I had a fiddler crab tank once... it was great.
It had a dry area and the crabs would fling each other into the glass with their claws sometimes when they would fight. You would just hear bang every now and then. They would warm themselves up under an incandescent lamp I had over the dry area and get energized and wave their claws around at each other and stand off.
The ones I had I caught from the wild, there's millions of them in the mangrove tidal zones, really easy to find and catch, they are everywhere if you go to the right place to catch them.