Yeah as IT GUY mentioned, hold off on adding your meds. If you're using copper bring it up very slowly over a few days. If you don't think that was the cause then freshwater dip before going into the QT can buy you some serious time and provide immediate but temporary relief for whatever the fish is dealing with.
Sorry to hear, always a bummer to lose fish specially trying to QT. I had a real bad run for a bit and ended up not QT'ing my current fish. It can be frustrating.
It's funny how that works, I've had really good luck with fish from Seaside in HB.