Hi.... I am not an expert here but your tank seems overcrowded in my opinion. Also, is the cat fish getting along with discus well? (Just curious)
If I were you, i'd move all the discus in to a bare bottom well established tank with daily water changes plus salt. This should do the trick in a week. If not, better you consult the experts here for medications. Also, better to grow out these discus in the bare bottom tank for few months before you introduce them back to your main tank.