Lets assume that there is no CI in the display tank (for sure).
Lets assume you treat with copper for 14 days at the correct level.
What is the reason you dont just put the fish in the display tank immediately?
Aren't the answers - 1. some people treat with another medication after copper. 2. 2 week observation after copper is because 2 weeks sometimes fails? There are articles suggesting that CI requires AT LEAST 2 weeks of copper. The protocol being used here is not standard - has failed once already with velvet (a higher level of copper is used now).
The point is not to criticize the protocol - the point is - I think @RtomKinMad asked a valid question. We cant be sure that a the person with the CI in the tank treated 'to short a time' for whatever strain of CI was in their tank. I think there is a lot we dont know about these parasites.
I cannot safely apply many meds in the DT. If some other symptom appears, I want to be able to have all treatment options available without having to destroy the rockwork again catching everyone. I absolutely, 100% want to do everything that I can to ensure that I am not introducing another problem into my DT or I may have to go fallow AGAIN... I'd far rather ride this out in a decent size QT until I am 100% positive