I haven't really had any luck with fire eels as they didn't eat, however i put a live earth worm but and tried to eat but too big to swallow which i couldn't find any smaller or skinnier as he was only 6" big.
When i do feel ready to move onto something other than cichlids, then is the time to buy a huge 12" or bigger fire eel and start off with actually getting him to feed even on frozen. I feel like i know what i was doing wrong with the last fire eel, that i didn't turn the light off after putting food in as i heard you should make sure and watch to see if they eat. He/she was very comftable in the tank swimming around but the eel did turn out with a few cuts from an cave ornament. I had sand but i guess i had to make sure there were no sharp edges on other surfaces.