starry night eel. get about 18" and stay very slender. Ours did fine w/ the same species, also never ate smaller fish ect. fire eels imo will deffinately outgrow a 75. I've seen smaller ones kept in 180's for life but they are a thick fish not just long. 24" is small ime for a fire eel...
Why some people have 4-6yr old fire eels that are only 18" boggles my mind all of mine have reached that size in under 2 years and kept growing. If you feed them bloodworms, and fish fillets they seem to stay small (stunted) if they get live blackworms, and nightcrawlers they seem to get 24"+ easy. They can and do get 40" in captivity if given the space/food. So to say they would be fine in a 75 for life imo isn't acceptable to me, some stunted specimins would be fine in that tank-size but I wouldn't suggest it as an ok size as a general rule.