Queen angel issue
 

I've had this queen angelfish in quarantine for about three weeks now. It eats a lot and appears fine except about a week ago it developed this small red spot on its chin. The red color went away but the spot swelled up. The fish still eats, but i am concerned that it may have an infection of some sort or maybe a gas bubble. My salinity is 1.020 and I have 0 ammonia 0 nitrite and 20 nitrate. I am also treating with .35ppm of copper. Should I treat with an antibiotic? Or will it go away on its own? Thanks.
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