[KodiakBear]

When I had ich in my DT I moved everyone to the QT with sand in it for my diamond goby. I did the hyposalinity treatment (which effectively killed anything living in the sand bed) but it took a few months to get the salinity down, stable for the designated time, then raised back up.
I just made sure to squirt some food mush into the sandbed near it's improvised den while in the QT to make sure it had enough to eat. He came out of QT happy as a clam, but carpet surfed a couple months later.

The other fish have all been ich free for 4 months now, and I could still use that sand since I didn't use any copper.
It is true that you can only treat ich at certain stages in the life cycle, that's why it is important to allow the DT to go through a COMPLETE life-cycle fish free, as well as treat the tank long enough for the little pests to develop into the stage where the treatment kills them.
Good luck!