When i kept Discus, i would feed them Tetra Discus as a staple, though i have read recently that most tetra foods aren't that good.
As a treat, they would get blackworms and beef heart. The beef heart makes a mess so like Joe said, water quality needs to be maintained.
I was always told to avoid frozen foods (Shrimp, blood worms etc) as Discus can pick up parasites from them easily.