You had no issues in this tank previously...?
Although you just added those 2 discus, you put them right into an established tank with other inhabitants. While the neons and bristle nose may have been fine, that doesn't mean they were or are a carrier of something that the discus could not ward of, or that they have never been exposed to and have fallen ill from...
You are also changing a minimal amount of water, and I get that the tank is lightly stocked but you are really going to need to get after those water changes, especially now to rid them of this...
If they are bloated you can try some epsom salts to help there...
If you have another spare tank I would get the discus out and in a smaller tank where you can do big clean water changes and get them away from whatever is in your current set up that has them sick.