When I purchased my RD about seven months ago at 5 inches, it ate floating pellets, krill, raw shrimp, flakes, and earthworms. Now at 11 inches lately it will only eat earthworms and will not eat anything else. Any comments?
My understanding is that this is very common in Red Devils, as well as Oscars. Providing your quite sure that your fish doesn't have internal parasites (no sunken stomach and no stringy white feces), consider fasting him for a few days and offering him pellets, that is of course if you're looking to change his diet to more pellet based (which I recommend).
Earthworms are healthy and Cichlids absolutely love them, but I think all fish are better off with a staple diet of a high quality pellet.
There are no symptoms of internal parasites. The diet has been the variety as described until he stopped eating anything for about a week until worms came into the food rotation and they were eaten like they are what he was waiting for. Shows no interest in anything but earthworms.