Well, the answer will depend on whether these are "red devil" Geosesarma
, which actually aren't fiddler crabs, or some reddish Uca
like U. crassipes
, which are true fiddlers -- but usually go by other names (like "watermelon fiddler crab").
If they're Uca
, they should be treated like regular fiddlers (brackish water and haul-out spots or dedicated land region). Geosesarma
are, as noted above, generally more terrestrial than fiddlers (but need plain, unsalted freshwater).