For me personally, I keep a 10 gallon tank running with copper at all times. I keep the ph, sg, and temp the same as my display. When I see a sick fish I take him out and put him in the qt. Since I set up the qt I onyl used it once. It was for my sailfin tang and fox face. I saved them both and still have them today. Except for the fox face, I took him back to the fish store to lower my bio load.
With all that being said, even if it is a fish only I would remove him. After all it cost much less to medicate a 10 gallon vs 55, 75, or whatever you have. This is just my 2 cents though others may disagree, but it has worked for me.