Calcinus laevimanus Hermit Crab (Decapoda, Anomura).

Calcinus hermit crabs are superb scavengers and will rid a tank of uneaten food and other wastes. They are one of the few creatures which will happily eat red slime algae (cyanobacteria).  They are found all over the Indo-Pacific in warm shallow water. Pretty peaceful, but may attack snails for their shells if no spares are provided.

Dwarf Blue Leg Hermit (Clibanarius tricolour)

Blue legged hermit crabs are superb scavengers and will rid a tank of uneaten food and other wastes. They are also partial to algae.

Blue legs are smaller (5mm to 8mm) than red legs but tend to be more aggressive to make up for it. Never keep them in a tank with Cerith snails as they love to use their shells as new homes!