I added a new yellow tang to quarantinne tank 10 days ago. It had no signs of ich until 8 days into treatment, all or a sudden it had ich! Copper can't kill ich in every stage of ichs life cycle so the fish should be in quarantinne with copper for 30 days after you don't see signs of ich and if you see ich partway in the process the 30 days starts over. That's how I do it anyway. And also, you need to leave your display fallow for 60-90 days to clear the system of ich. I left my display tank fallow for 4 months, so far it has worked great for me. No signs of ich on any fish in the display!