Most of us dont QT our fish, we rely on keeping our fish stress free and feeding a quality diet to keep them disease free. Its very very easy to keep your fish stress free and healthy, just follow three simple rules 1. Dont over stock your tank unless its well matured and you have planned ahead to do so! 2. Dont buy fish that cannot live their whole lives in the size tank you have! 3. Go slow, nothing good happens fast in this hobby.
I cant think of a single person on this site who has followed those rules and has a problem with disease in their tank, and I can also think of several people who havent followed those rules and have had numerous issues.