Well I have a 125 gallon that im setting up. Its 72x18x22 L/w/h. I hadnt quite decided what I was gonna do with yet. But my friend has a 100 gallon 72 x 18 x 17 but I wasnt sure if that would be to small or not.
I think you' d be fine either way. Fire eels are big, but they aren't really swimmy. They usually cram themselves under sand and décor then chill. so aslong as they have a nice soft lounging spot, big enough to comfortably hide, you' re good.