Q/T question

Just started a q/t 2 weeks ago because my blue hippo tang got some serious ich. My q/t is a 20 gallon long with just a hang on back filter with just a floss pad and a small heater that keeps the water at 76 degrees. I used water from my main tank for it. I added my small blue tang into it and slowly dosed with with copper over a week tell i got the correct amount of copper in my q/t. The blue hippo tang has been doing fine for two weeks now in q/t. Then I noticed that not only did my hippo have ich but the other fish that are in my main tank have some ich not as bad as my small hippo did though. So I added my other fish into my q/t after acclimating them to the copper. Now all of the fish I added with my hippo tang in my q/t have a lot more ich spots, way worse than they did before going into the q/t. Is this a bad thing or pretty common from the copper and new tank?

 

while I have never dosed copper, I do have some clarification questions.

1. was the filter pad in the HOB filter used in your main tank at all before putting it into the QT? If NOT, then you have very little biological filter in the QT (unless you have rock and such)

2. How many fish are we talking here?
3. how big is this blue tang?