Help 5x discus sick!!

Hey guys,
I recently added 6 new discus to my established 4ft tank which already had 5 discus. After a couple of days the old discus started turning dark and have white secretions, it's really hard to describe but I have attached a picture. They have been looking like this for over a week now but the new fish still have no symptoms what so ever!
I have no idea if this is a bacterial or fungal infection? The fish don't seem to be getting any better or worse, and they are still all eating so I am hoping that something I am doing is helping.
So far I have raised the temp to 31 degrees done a water change and added Bactonex and have just added a second treatment of Trial Sulfa on the advice of an aquarium. Is there anything else I can do to help them? Im worried they will all end up dying!