That tank could be a little larger but electric eels are funny creatures and don't actually like having too much room, they seem to require constant contact with objects in the tank to feel really secure and will fade out and lay on the bottom when given too much room. A guy i know who used to look after a electric eel at a zoo in the states said that every time they tried to move their eel into a new larger enclosure it would keel over and they would have to rush it back to its old tank to keep it alive, in the end they had a 5 foot long eel in a 8 foot long tank despite the fact they had prepared a big 1000 gallon tank for it.
Both PaulMTS and myself keep electric eels, pauls is being moved to a 200 gallon tank today and mine is in a 80g and will be moved into a custom built 60x32x12" tank later this year.