I am a firm believer that ich is NOT always present in a system. I had many losses (butterflies/tangs) when I first started in the late 80's. I could always trace it to a particular introduction. I had ich once in my previous tank, introduced by a royal gramma. It was getting progressively worse, even an established springeri pseudo got it. I attached my UV, and it cleared up in a week. I'm making no claims here, just relaying a story
I have had no ich in my new tank. A attribute this to 1. Most of my fish are anthias, which I personally have never seen contract ich. 2. The tomini tang was at the dealer for 2 months. Even if he carried ich, he didn't show it. And he just carried it into a tank full of established anthias.
My final fish for my tank, a powder blue tang, is in QT right now. He was mail ordered. He shows no signs of any problems, but with their reputation, he will, nonetheless, remain in QT for at least 4 weeks.