Here’s a thread I have here. I went through the same thing. I had many different seller discus. Was lucky for years. Than I got a cross contamination (cc). https://forum.simplydiscus.com/showt...ing-for-help!!.
As to your question. It’s a good one. Never been there. But would guess the tank with the now sick hero fish you would heal the sick fish. Medicating the fish with it thus killing whatever it is they have too. Now all being healthy they should be good to go. Or start a new tank for the new ones and the hero.