There are a number of things wrong with your tank setup.
1. DPs do not belong in it at all, They belong in a species tank. You will lose other fish to them, even bigger ones.
2. Discus dont like bright light.
3. Discus dont like much flow, you have a lot.
4. Discus need very clean water of at least 82 degrees.
5. What do you mean by sucker mouth catfish? It could mean anything and some of them will go after discus and/or other fish.
Here is what I would do to treat the discus. Put it into an H tank. get the temp up close to 90F. Be sure you have good aeration. Add 3 tablespoons of the Epsom for every 10 gals. Dose the tank with Maracyn or generic Erythromycin following the lable directions. Do not add salt. Change 25% of the water every other day, after the change dose that days meds and be sure to replace the 25% of the Epsom you removed with the water change.