First off, all fish should be quarantined before being put in your tank. In the event that a fish comes down with ich, there is a good chance that the other fish in your tank will get it as well. If your fish don't seem to be getting it, then you at least know that the ich is present in your tank. Ich cannot survive without a fish host, so allowing your tank to go fallow for a month would give it time to die off.
When fish are stressed, their immune systems weaken, thus giving the ich its opportunity. To reduce the overall stress in a tank, you want to make sure all of your fish are compatible with one another (no aggressive behavior). You also want your water to be of very good quality and consisting of the proper parameters. Make sure all of your fish have plenty of places to hide and space to call their own.
The fish would be less stressed without the ich. And if something in your display were causing the stress, it's a good chance that putting him in the QT would in fact make him less stressed (there are occasions where moving an extremely stressed fish is the last thing you want to do unless death is imminent). The salinity of the QT, if you are not performing hypo and are treating the fish in some other way, should be a few points lower than the standard (34-35ppt or 1.026 SG). This is another stress reducer. If you don't feel comfortable lowering the salinity for reasons other than hypo, it isn't a necessary component.
Hope this helps!
-lefty
