In the wild they eat certain plants (algae), live fish and live worms. Feeding veggies will however be difficult. There are many videos of large eels taking large chunks of fish as well.
Mine eats night crawlers, market shrimp and tilapia. I've out of curiosity thrown cod, silversides, halibut and a small piece of catfish to see if he'd eat it and he did. Those were just tests once. Guess he'll eat anything I throw in.
Mine is only about 12" but I feed him frozen Mysis Shrimp and Krill. He eats the Mysis out of my hand, but won't eat the Krill til I drop it...weird, I think he doesn't want to bite me on accident. At least I tell myself that, cause I love him.
I have a 2.5 foot long black spotted eel in with my big bichirs. The eel will only eat market shrimp and tilapia the silversides and beef heart that the bichirs love the eel won't touch. He is a big eater. Will down a whole handful if shrimp on his own.