Yes that's what you have to do in my opinion. Ich or Velvet is in your system and it'll keep coming back to kill your fish. I have a tang Gang so I know exactly what you're going through. I would just treat the main display tank now and then stick to a strict QT Process after. I did it in my tank. I now Quarantine everything and absolutely nothing goes directly into my display tank. I Quarantine fish for 90 Days and Coral and inverts for 60 Days.Then try to take the copper out after a month or so