You need to get it out before any of the cysts come off, but it would not be wise to think that your tank is not contaminated with ich.
There is no way to treat your display as a whole.....but ich only affects fish, so......
You need to make a decision as to how you are going to handle this.
1) Remove all fish to a QT setup, treat them with either copper or hyposalinity. Keep your display fish free for 6-8 weeks, so that the ich dies off in the display.
2) Try to keep your fish healthy, feed foods with garlic. And don't keep any fish that is a known ich problem. (Most tangs)