They don't need brackish.
I've had a male and female fiddler in my fresh tank for about...two and a half, to three months now.
Not to say there is no salt in the tank, theres some there to keep the fresh water fish happy, but its far from brackish and my crabs are thriving.
I do have a toy fireladder in there from a childhood toy for them to climb up onto to get above water. and I've enver had one escape on me.
Had them get in strange places, like inside my filter once
But never out.
and my fiddlers eat just about anything that any other bottom feeder eats.
They'll pick algae off rocks and plants, I swear they eat the plants themselves, they eat shrimp pellets I put in there for them and the like.
I've never had them get hold of one of my fish, I've had a fish die and they get to it AFTER it died, but never kill one.
Also do good, for the most part, with other bottom dwellers.
I have a
CAE and a Peacock Eel that sit on the bottom alot, and they never once bother any of them