Also, if your end up putting the fish in a QT and do the copper treatment, You will want to leave the display without fish for at least 8 weeks. That is the usual length of the life cycle of the parasite.
You may know this already, but never treat the display tank.
For future, you may want to invest in a full-time QT tank. This way you can QT any new fish and catch any diseases without introducing them to your main display.
I don't believe you will truly get rid of the ich without QTing the fish and treating them.