Or... he is eating the slime coat of the bichir. Some people love rainbow/red tail sharks, but I find them to be aquarium nuisance. Very aggressive for no reason. A 2" one bullied some 8" goldfish I had and bruised them horribly. I could be wrong but it seems to me your shark is just eating slime coat. Bichir are pretty tough fish, but I doubt he will make a move to eat the shark... to fast for him.