Nudibranchs, often called sea slugs, are a fascinating animal. They do not live long on the downside and could also poison your tank if they die. The best one to get is a lettuce sea slug. They will not pollute your tank and will clean up hair algae with ease. They are difficult to keep but overall, are a neat animal. Experienced aquarists should get one of these, although some fish, such as triggers will eat them.