When you have ick on fish, I agree that the best advice to eradicate them is leaving QT with no fish for 8 weeks while treating fish in QT. What I am gonna relate next is what I have done so far without removing all the fish from DT and with success. Just remember though that ich will start to proliferate only if you have fish that is stressed out and becomes weak. Stress can be caused by either being harassed by other fish or poor water quality. It usually happens when introducing a new fish in your tank. The most susceptible ones are tang due to their thinner protective skin coating.
The first sign of ich in my tank was when my Blue Regal Tang was covered with white powdery thing in it's body. I moved him in QT with copper treatment for 6 weeks. However, while my BRT was under treatment, my yellow tang started to show some sign of ich also when it keeps scratching its side in the rocks and substrate. I started treating the DT with Garlic Xtreme 2x a day and also mixed the frozen food with same, plus other food supplements. The scratching started to subside until it was totally gone. There are other fish such as flame angel, clown and damsel which were not affected. It's already more than 4 months and my fish are happy with no more sign of ich. My BRT is on separate tank cause I feel it's not comfortable being with yellow tang.