For these species, a brackish tank with at least one haul-out platform (sloped emergent sand preferred) is required. Try to maximize submerged structure (caves, driftwood, and the like).
I have a 10 gal tank. had a redclaw & fiddler. redclaw ate the fiddler lps said 10 gal was big enuff 4 both, maybe 4 2 fiddlers, or my redclaw was really agressive hope this helps
im my brackish 75 i have 3 male fiddlers and one maryland blue crab. the maryland blue crab dug himself a burrow and lives under a decoration. the fiddlers are always out in the open or perched on a floating plant or the heater so they can haul out of the water. I've had them for about and month and a half and they are all doing great. I have only had the blue claw for a week and he's a lot more agro than the fiddlers. Two of my fiddlers have molted already and I saved them because they looked cool. I am surprised they survided considering I have two figure 8s in the tank that killed 4 crayfish, lots of feeder shrimp, and snails but leave the crabs alone. I can't wait til the blue crab molts and gets like 2 inches long, because he will look awesome and all.