Shrimp and crabs are the staples of many reef cleanup crews, and they are very useful in our aquariums. They clean up uneaten fish food, nibble on algae, and even remove pests before they become an infestation. That said, these opportunistic inverts can also become a headache themselves. The hermit crabs can attack and kill snails, shrimp can devour coral polyps, and large crabs can eat anything and everything they want. So, introduce crustaceans at your own risk, and make sure to get the ones that have a better reputation like cleaner shrimp.