I faced a similar situation last Dec. I set up a QT and treated my surviving fish. They are doing great now and about to return to the DT, which remained fallow for over 76 days.
I am so glad I did. Fish are much healthier now and I feel better about it too. From now on--everything wet gets quarantined.
If you can't treat them in quarantine then cross your fingers because that's the only way to get rid of it. You can set up a 10 or 20 gallon QT tank cheap.
I'm not one of the many disease experts here, but if you do not intend to remove the fish and treat, and then let the tank run fallow (fishless) for 76 days, you may want to consider reading through the thread below.
Others will chime in but my guess is they're going to advise removal and treat in QT/hospital tank.
It's a pain but unfortunately, if your tank contains any inverts, your only option is to move all of the fish to a quarantine tank for treatment. Otherwise, they will all catch ich.
Your main tank will need to remain fishless for 76 days until the ich has died off. Sorry.