My tank cycles completely, which surprises me because it's a tank of only 10 gallons that I put for small fish, my damsel came with velvet so I treated her immediately, I was only surprised to see her very stressed and without wanting to eat, for a easy to keep fish that is not normal, did not know about copper resistant bacteria either. Solve the problem with water changes, but a delicate fish and perhaps without having time to change the water would have died immediately.When you cycle a tank and then add copper, it’s normal for some nitrifying bacteria to die. If you didn’t cycle the tank long enough you might even get an ammonia spike.
Since I sterilize my QT every batch, I really don’t have the patience to wait for a cycle. Doing the method with cycling media in copper fixes that for me.