Eels

Eels are a unique and hardy family of fish that actually adjust pretty well to life in an aquarium.

They generally prefer that their entire body be covered, so extensive live rock with plenty of gaps for them to hide in is required. Some hobbyists use cut pieces of PVC pipe to ensure that they have a tunnel to hide in. Eels also require a tight fighting lid as they will try to escape frequently through any small gap that they can find.

Though they will ignore coral and anemones, they'll pounce on and eat any fish or invertebrate that is small enough to fit into their mouths.