Not all Hermit Crabs are considered to be safe for reef tanks. Many Hermit Crabs will destroy corals, eat fish, or other invertebrates. Growing Hermit Crabs are always on the lookout for a larger shell to settle in.
These hermits are considered to be reef safe janitors.
The Dwarf Yellow Tip Hermit Crab (Clibanarius sp.) is a great addition to a saltwater aquarium because it eats many kinds of algae, including red slime algae (cyanobacteria), and sifts the sand, as well. It's small size (to 1") allows it to get into all of the nooks and crannies in a reef tank.
The Halloween Hermit Crab (Ciliopagurus strigatus) is a great addition to a saltwater aquarium because it eats detritus, uneaten food and many kinds of algae, including red slime algae (cyanobacteria), and sifts the sand, as well.
The Electric Orange Hermit Crab (Calcinus sp.) is a great addition to a saltwater aquarium because it eats detritus, uneaten food and many kinds of algae, including red slime algae (cyanobacteria), and sifts the sand, as well.
The Electric Blue Hermit Crab (Calcinus elegans) is a great addition to a saltwater aquarium because it eats many kinds of algae, including red slime algae (cyanobacteria), and sifts the sand, as well.

Savdy PhouThe Dwarf Blue Leg Hermit Crab (Clibanarius tricolor) is a great addition to a saltwater aquarium because it eats many kinds of algae, including red slime algae (cyanobacteria), and sifts the sand, as well.

Photo by Craig SmithThe Scarlet Reef Hermit Crab (Paguristes cadenati) is a great addition to a saltwater aquarium because it eats many kinds of algae, including red slime algae (cyanobacteria), and sifts the sand, as well.
The Dwarf Red Tip Hermit Crab (Clibanarius sp.) is a great addition to a saltwater aquarium because it eats many kinds of algae, including red slime algae (cyanobacteria), and sifts the sand, as well.

Photo by Joe MartinThe Zebra Hermit Crab (Calcinus laevimanus) is a great addition to a saltwater aquarium because it eats many kinds of algae, including red slime algae (cyanobacteria), and sifts the sand, as well.