Crabs in the reef
Some crabs are useful scavengers in the reef. Others are predators that attack corals and fish at night. The choice must be made with caution.
Detailed explanation
Emerald crabs (Mithraculus sculptus) are reef safe and eat bubble algae (Valonia). Porcellanid crabs (Porcellanella, Neopetrolisthes) are harmless filter feeders.
Many crabs are opportunistic predators that attack at night. The Sally Lightfoot crab (Percnon gibbesi) is reef safe when young but can become aggressive as an adult.
The general rule: if you're not sure it's reef safe, don't put it in the aquarium. Crabs often arrive as stowaways on live rock.