Foxface breathing hard (panting)

[dusty16]

Ive had a foxface for 2 months now that has been sitting in quarantine since I bought it. Everything was good until this morning when I noticed that it was breathing really hard. 2 clownfish are in the tank as well and appear to be fine.
Tank parameters
Salt - 1.024
PH - 8.0
Temp - 78
Nitrite - 0
Nitrate - 3
Phosphate - 0
Ammonia - 0
I have a HOB filter and a powerhead for water movement.
I just removed copper treatment a few days ago (3 week treatment using cupramine) so parasites shouldnt be an issue. Nothing visible on the body.

Any ideas as to why its happening and what to do about it?