Invertebrates in the aquarium bring a whole lot of perks to the tank. For one they are often excellent cleaners that make maintaining your tank much easier. Additionally they do not add to your bioload as fish do.
Be warned however, some invertebrates are aggressive, eat corals and can become real nuisances. Look up your new critters before adding them to the tank. Some are impossible to remove once they hit the water!
Snails are typically seen as clean up crew members. They slowly move around the tank and eat the waste/algae that they find. Some are better cleaners than others and some are even predatory.
Shrimp are the secondary cleaners. These guys often run around the tank fairly quickly and eat algae and left over food. Some will even form symbiotic relationships with other fish. Others can hunt other shrimp, snails and fish. Unlike snails these can add a lot of personality to your tank so be sure to add a few.
Crabs are similar to shrimp but are slower and stronger. Many make excellent cleaners as their strong arms can rip off even the most stubborn algae. That being said they possess the power to tear corals and hurt fish. Even small hermit crabs can be dangerous. Add crabs to your tank with caution and be aware of their maximum size.
Only for large tanks, sea stars make fantastic cleaners and sand agitators. Scouring the tank for foods and crawling under rocks, keep these only when you are 100% sure you have the right conditions. Sea stars have one of the highest mortality rates of any invertebrates.
These little shell-less snails have a wide variety of traits. Some are fantastic clean up crew members while others are accidents waiting to happen. For those with problematic hair algae sea slugs can be the answer. Just be aware of the needs the slugs can have.
A common hitchhiker, the fan worm is a stationary filter feeder who is often seen as aquarium eye candy. Their body is segmented and they will form a tube to live in. They attach this tube to rocks or sand. They can reach up to 10 inches in diameter, making the fan worm one of the best decorating inverts.