I'm no help with the knife fish, but I've kept a fire eel before. They do take a while to get confident enough to eat, but when they do you'll wish they hadnt!! Mine loved bloodworms and refused to eat anything else [5 cubes a day when it was about 14"], but I've also seen them eat shrimp. They're really easy to hand-feed, and IMO it's the best way to make sure theyre getting food especially with other fish in the tank.
Good luck!!