Sick or new fish need one thing above all else- clean water. Asking them to heal a wound, or battle an illness in less than perfect conditions is counter-productive.
I would change out a percentage of the water every day.
That being said- if you are medicating with copper then you have other issues- like maintaining a specific ppm at all times. If you change the water then you have to test and adjust it- every time. Between water changes and evaporation I don't think it is even possible to adequately treat with copper without testing at least once per day.