Frozen Blood worms, live and frozen Brine, black worms, small ghost shrimps etc, several brands of prepared foods, frozen beef heart, earthworms etc.
We like variety, so do fish, so offering them a wide range helps. Frequent feeding helps also.
If you have young fish, you might consider growing them out first in a bare tank then adding them to the planted tank later if "size" is the main goal.
Good feeding is more meaningful than just about anything else here.
Several companies make discus frozen cubes also, these are good, but a mix of several things and live brine etc are always good.