To kill ich, salinity would have to go down to 1.009 for a period of a several weeks, and the salinity would have to be dropped rapidly enough that the ich doesn't acclimate. And of course, you can't keep the salinity at 1.009 for too long because the fish can't handle it anyway. But if you do it in qt, then move the fish into the dt, you would have erradicated the ich anyway (assuming your other fish in the tank have also been treated for it, otherwise it's a moot point to treat just one fish), so having the salinity be at a normal 1.025-1.026 is best.
You should try and identify why you have an ich problem, fixing that usually solves the ich issue. Most often ich results from the fish being under constant stress, usually from being in a tank too small for them, too many fish in the tank, or inappropriate tank mates