u can add copper to display tank if u intend to go fowlr. copper will kill every single thing in your tank except for fishes. ich is still in your tank, no matter what fish u add, they will still get ich n die.
for reef tank, simple method, do water change often, dose bacteria, wait till ich is "gone" then add in new fish. One at a time.
hardworking method, catch out every single fish, even fishes that are not affected by ich, leave the display tank fishless for at least 2 months. meantime set up a Q tank and treat your current fishes with hypo or copper.