Sturdy rock piles--eels tend to spend most of their time in the rock work, and are strong, so you need to make sure they won't accidentally knock loose rocks. Limited other fish--ones that won't be seen as a meal, might be aggressive and hurt the eel. You also want the tank to be well covered--eels are notorious for carpet surfing fatalities.
as OrionGirl said, lots of hiding places, and a well covered tank. if you would like to see your eel a lot, u could lean a piece of slate against the front glass before u put in any other hiding spots, then your eel will hide in plain view