I recommend feeding them a varied diet consisting of meaty foods such as brine shrimp, bloodworms, and beefheart. However, when you feed bloodworms, feed very lightly. If they eat too many worms they can get constipated and then they're really not very good after that.
If those foods are unavailable to you, or you feel they're a bit expensive, you can also try a mix of high quality flake and pellet foods, and in that mix be sure to get some spirulina flakes in there, which is insanely nutritious for discus and really any other fish.
Discus can also be "trained" to eat lettuce, zucchini, carrots, and peas. Don't leave these vegetables out of their diets. They enhance color and vigor, and are all around great for discus.
For young discus, feed lightly four or five times a day. For older discus, feed a larger quantity once or twice a day.
