Hi Alex, I can see the pictures. The one discus which is really dark is pretty bad and if it survivals the meds it will be one lucky fish. The gray stuff you see is it's slime coat. Unfortunately all the fish are thin and that has hurt their growth and health. Slowly reduce the heat to 84 or so. Dealing with the high heat is stealing valuable energy they all need to help recover. I think mostly these fish are dealing with a cross contamination and generally poor condition. Complete the Prazipro treatment and if the one bad fish is still around and losing it's slime coat I would suggest Furan 2 with salt. Adding salt now will help them and start with five tablespoon per 10 gallons. Add back five tablespoons with each 50% water change.
In the thread you linked ignore the advice to add junk to the tank.
Pat