I am actually going through the same thing. My powder brown tang had ich and infected my DT water. Unfortunately, you will have to treat all fish in a QT and leave your DT fallow for 72 days. The reason for this is because the ich parasite needs a fish to host on. It's life cycle needs to complete before your DT water will be ich free.
Trust me, even if the others look fine, you need to do the treatment. I will be starting Cupramine treatment tomorrow (Copper). Copper apparently is a good treatment for this. However, some fish like angels, should not be exposed to copper.
