I have a few in my "assorted" tank, and I feed them the same thing as the rest of the fish in that tank. Flakes in the morning, hikari cichlid pellets in the evening.
Sometimes I swap out some freeze dried shrimp or worms or something in the evening. The bichirs eat anything I put in there...

I don't feed anything in that tank feeder fish, although there are many in that tank that would eat them. They seem to be thriving, as mine have grown from 4 inches to 12 inches in a little over a year.
Good luck!