Blue Legged Hermit crab
When it comes to keeping the tank clean, Blue Legged Crabs are a good choice. They keep the substrate stirred up as they meander throughout the tank, and they can consume some algae if it is within reason. They are known to kill snails to steal their shell for themselves. After all, these crabs do grow, and their home becomes too small. Reef-safe.
Photo by Tod Bogard (buzz123)