Remove all to qt and treat qt with copper. Or leave all in DT, limit stress, feed well, and hope for the best. I have done both methods. I have dealt with ich recently and did not remove anything and did not treat with anything. I increased feeding and removed a fish that was causing some tension and I haven't lost a fish. All are fighting the ich off and hopefully will continue to do so. I have a 200+ gallon system and removing the fish ain't happening. Some frown on my decision but there have been recent threads on people crashing there tanks killing off fish, coral, and inverts because they were trying to remove there fish to treat. Other posts reveal multiple fish laying on there side in qt due to treatment meds. There are risks either way. I have treated in qt, left tank fallow for months only to end up with ich so it's not for me. Best of luck but there is no sure method to treat DT that will remove the ich but not harm inverts, etc.