You could also try feeding it live fish (that are small enough for it to eat of course), brine shrimp, bloodworms, large worms/black worms, beef heart, krill, prawns, & other freeze-dried or frozen meaty foods. Some will even eat flake food and sinking catfish pellets. It is said you should always include live fish in their diet; but don't make live fish its only meal all the time (always feed it a variety). If you want it to grow as fast as possible I would feed it a variety of foods, do water changes every week or two weeks, and provide it with a home that is plenty big. Fire eels will get around 20-32" in captivity but it all depends on how big the tank is.
Other info I found out online that you may find helpful,
"a soft substrate is necessary for this fish. The Fire Eel will often dig in the substrate and bury itself; this may uproot plants and rearrange decorations. A tight lid should be used for any long, slender fish, as they will often try to escape from the aquarium. The Fire Eel should be kept with fish of similar size or larger, and some will not tolerate their own kind."