Well if he had stated that he was getting swinging changes in parameters, then we would have already known that he has a tank that was not properly cycled.
I'm sorry to say, but as much information is at your fingertips and assistance always at the ready. Yet you feel that you need to rush into things and want to drop a fish immediately, the only words that I have to say is.... good luck.
Hopefully everything turns out well with the tank, and the eel does not suffer too much.
Oh and BTW... the fluval filters and the like are a poor means of filtration for SW tanks. You should have done some research on canister filters.