I am sure this is wrong but I forgot the reason. This was my line of thought: Ich stays on the fish for only a couple of days and then falls off to form a cyst. In addition, copper kills any free floating forms of the parasite so re-infection in copper shouldn't be possible. If so, why not keep fish for one week in copper QT and then transfer to a clean tank? Why is that not sufficient?
Cheers!
Cheers!