Feed whatever high quality food it will eagerly eat,wkly water changes,temp 76-78(lot of people keep it higher but it isn't good for the long run,speeds up metabolism and shortens like span if 80 or above,long term)
I understand that while the fish are young you should feed them a lot of protein which includes krill, worms, crickets, feeders (clean). You should also feed two or three times a day. When the fish get older you should reduce the amount of food and number of feedings. This has worked well for me.
quality, high protein diets, for growth, nothing to fatty, no feeders. good water conditions, water temp - 77-78.
they are fairly fast growers anyway, what size tank is he in, you could do him in a 75 for life happily, people keep them in 55's which is ok but for a best quality fish, bigger is beter.
"hikari bio gold+" will make your devil grow huge , small water changes every other or every three days I find make fish grow faster because the excess nutrients never get to build up and the fish are rarely exposed to them even for a short amont of time.
use the bio gold +. as said he will grow like a tank, they sell in tiny pellets which i feed to my growouts which they love and they grow pretty fast for there average growth rate.