Another one for the Kusuri Discus wormer! Its fantastic stuff! I used it whenever I added new discus or angels and as a rule, I used it in every tank twice a year. Never had a problem with parasites except on new fish and the Kusuri fixed it. You can get it on ebay at a good price too.
Also, is the smaller discus getting picked on by any of the others? Discus are notorious for their bullying of smaller individuals...
And thirdly.. what are you feeding them? They need more food than your average tropical fish do and a good mix of a staple pellet food (Prima, discus granules, Novodiscus etc) and a good mix of frozen foods (bloodworms, krill, mysis, brineshrimp/artemia, black mosquito larvae and small quantities of beefheart). Garlic is good for them too, you can buy liquid garlic in the marine section in most LFS that have marines and add a drop into a cup of frozen foods that you are defrosting so it soaks up the garlic...
They also love little earthworms etc too if you can get them..