Eels are incredibly majestic and thrive in most conditions.  They are most commonly seen in areas with an abundance of live rock.  With their snake like shape, Eels can maneuver in and out of rock holes, caves, and caverns with quick reflexes.  They eat small fish and invertebrates, so it is best to surround them with larger more compatible fish.  Eels do not eat coral, so they are certainly an option for coral reefs.  The main things to consider are that some Eels can grow very large, and all Eels will require a tight fitting lid for their environment.