NO SALT. No pH ADJUSTER. NO NEW DISCUS. Increase your water changes to 30% or more 3x per week. Test your water right out of the tap for GH, KH, and pH, and then again after letting it stand for 24 hours, and report the results back here. Make sure your water is treated with chlorine and not chloramines...there is a big difference and you use different neutralizers depending on which you have. The salt, pH adjuster, and water softening "pillow" are all making conditions in your tank worse, and your low % water changes are allowing organic pollutants to build up to a level where they are irritating the discus and impairing their immune system as well.
If your discus does not improve with improved water conditions, you may need to try an antibiotic, but that would not recommended as a first course of action. If you do wind up going this route, then levamisole (available, I believe, in a product called Disco-med) would be your best bet. However, your problems are coming from your water conditions, and until you get those resolved nothing else will help.
I also highly recommend that you get a book that deals in detail with water parameters (GH, KH, pH, etc.), as well as a book about discus, and read them both before investing in any additional fish. Tetra's Guide to Maintaining a Healthy Aquarium and the Handbook of Fish Health are both quite good, as are all the volumes of Baensch's Aquarium Atlas (volume one will have the most basic info, though).