As peacockmadness said, I own a fire eel and I house him in my 220. (6x2x2.5) he is a little over 12", and ive had him around a month. I dont think one could be housed in a 125 for life. Even if the tank has no decor, and the eel is the sole inhabitant, the footprint will not be appropriate. People justify smaller footprints by talking about how flexible they are, I say just because a contortionist can fit in a suitcase doesnt meant they are happy being contained in one. However I think that the eel would be more than happy in the 125 until 18"-20" in size. I have wanted one of these amazing fish for a long time. But until I upgraded to atleast a 6x2 footprint I wouldn't have even thought of housing one of them long term.