The clown was already infected, long before you put him in QT and started meds. The transfer and the meds stressed him and lowered his immune system, likely. Keep the qt water stable and provide a low stress environment. Low light, not a lot of movement around the tank. I wrapped the sides and back of my qt with a dark aquarium background.
I bet adjusting the salinity that fast stressed him too. Never do anything that fast! Everything must be done super slowly.
I would think since he didn't show signs before you QT'd him that he'll be fine if you keep everything stable.