I doubt anything smaller than a 55 or so would be good. My first tank was a 58 Reef Ready ( about 3ft long) and the eel in it is currently about 3yrs old. He is two feet long easy. He is also thicker around than my thumb. VERY hardy critter but even as a "small eel" they get pretty sizeable. Granted they stay up in the rocks, but having an eel longer than your tank just sounds like a bad idea to me.