The green wolf eel (Congrogadus subduscens ) is not a true eel, but rather is a member of the Family Pseudochromidae (dottybacks).
They get to about 12" and I seem to recall reading that they get somewhat nasty; both in terms of not liking to share intimate spaces with other fish, and with a tendancy to snack on smaller tankmates.
A fairly interesting fish, but one you have to be a bit careful what you want to put with it. Generally not considered a "reef" fish due to its dietry nature of eating tankmates small enough to fit in a fairly large mouth.