P. endlicheri, try to get on a quality pellet (I prefer NLS, hikari, or omega one), earth worms, black/tubifex, tilapia fillets. occasional bloodworms, silversides, prawns as treats.
Careful with the gravel, not only is it a detritus trap, many polys ingest gravel and suffer impaction which can be fatal.
I have a tube in the tank that I put the food in. You might have to make the tube smaller so the cichlids don't get in it. Or you could move the gravel to one side and put the food on the bare bottom.