3 weeks into copper QT and black patch

[yishi]

Hi,

My new Blue Eye Tang in 15 gallon QT tank 2.2ppm copper power was eating fine for 3 weeks and today I noticed he has developed a black patch. Still swims OK but losing appetite.
The other tang and foxface in this QT tank are fine.
What does this look like? Aggression damage or bacterial?

I started prazi a week ago.

I have quarantined more than 6 fishes using the same protocol in the past for 30 days and never seem this before.

Thanks.

 

[Subsea]

Two tangs and one fox face in 15G QT tank seems like a lot of fish biomass. How do you control ammonia buildup in this small tank?