Mine eats flake, pellets, et cetera. He is in a community tank, but I always drop some big sinking pellets at his end of the tank. I feed just after lights-out and he comes right out. When the lights come on he crams himself in a cave if he's not already in one. They also like small or chopped up earthworms. Basically they eat everything.
As an update my experience with the niger has been a fairy slow/medium growth rate. I got him at around 10" and he is now in the 16" range and very healthy. I also upgraged the tank and he is in a 300G.
I don't overfeed my fish and he does mainly cleanup by my experience has been a little over a half inch a month growth.
