I have a large tank with pretty big fish, so I can't QT very easily. It is always better to have a QT, but if you must treat a display, I have had reasonable success with Paraguard by SeaChem (malachite green and some kind of aldehyde). It's marketed as an alternative to formaldehyde. I left the skimmer and rock in the tank, no carbon, and treated daily at the recommended dose of 5ml per 10 gallons (it adds up in a big tank).
I had some weaker fish, which I was able to net daily to give 5-minute fw dips followed by a 1-hour, more concentrated, Paraguard bath (3 ml/gallon). I lost one fish, but the other six fish survived and have done great since.
I have done this with another large DT, with similar, pretty good results. Though in my case, I believe my fish had velvet, which is quickly fatal, and does not respond well to hypo. With ich, generally, hypo is a good control.