If you've got a tank with smaller fish, and some patience just buy some amano shrimps as soon as you can.
They grow to a good four or five cm long, and are pretty chunky (especially the females). They live for many years after they reach that size. (My biggest one (named Moth-Ra) is about five years old.) Actually, there are two Moth-Ras, both females, same age age, and both still constantly covered in eggs. I wish I had the time and patience to raise the eggs, but all that salt water malarkey is just too much for me. Also, I'm sure the fish appreciate the regular feasts when the poor little blighters are released every couple of weeks!
Try and see some mature amanos if you have any mates who keep them - I have never kept discus, but their mouths don't look that big to me.
Mark