^I’m also wondering what type of copper and test kit you’re using.
Have you tested ammonia, nitrite, and nitrate in your QT since removing the rock? Also, any aeration besides the one powerhead? Sometimes just throwing in an extra air-stone can make a big difference when using medications...
If the other fish in QT is doing well, I’m more likely to think it’s something parasitic (like the internal parasites already mentioned), but since all fish handle stress differently it’s good to cover all your bases. Sorry your having trouble with him, hopefully you can get him turned around...