Red Mithrax crabs, Mithrax ruber, are very close relatives to the green emerald crabs, Mithrax sculptus . The red mithrax crabs tend to be more of an offshore, reef dwelling crab, so I think they are actually more suited for a reef tank than the green crabs, which tend to be more of an inshore, non-reef dwelling crab. Both crabs will clean bubble algae, and hair algae, and both will clean other unwanted fleshy macro algae from your tank. You can mix them freely with the emerald crabs and they will get along fine. Maximum size is about 1.5 inches carapace width, and we prefer to ship them at about 0.5 to 0.75 inches.