I have two big fire eels, and they are crazy little buggers. they share a 6" PVC together, and attack anything that gets close. They only come out when its time to eat, and even at their current size they are still extremely finicky eaters. When they are hungry enough though, they don't let anyone get in their way. I've seen one of them rip a worm right out of a big oscar's mouth.
That's just mean.. its no different then putting a Oscar in a 30 gallon for it whole life!