I would get her to eat tomorrow if you can, using different fresh foods (squid, white meat fish, etc) dipped in garlic supplement if you have any. If you can get her to eat before you begin treatment, the odds are that much better the fish will survive.
As for copper, I use Cupramine. The directions on the bottle are pretty easy to follow. The only thing I would change would be to lower the dosage to .3 mg
The basic formula is:
(.381) * (dosage level) * (gallons of water being treated) = mL of Cupramine to dose
Example: (.381) * (.3mg/L) * (20 gallons) = 2.3mL of Cupramine to dose
In this example, now that you know you need 2.3ml of cupramine for the 20g QT tank, I would dose cupramine over 2 or 3 days to get the total dosage needed. In this case I might dose 1.15ml on the 1st day, and 1.15 ml on the second day.
As you perform partial water changes, you would need to add the correct dosage to the QT again since the new salt water you're adding to the QT would dilute the copper. To determine the PWC
change needed, check your nitrates and other levels as needed.
Example: after checking your nitrates, you determine a 25% water change is necessary. You would remove 5 gallons from the QT and add 5 gallons of fresh salt water. Currently, the QT is duluted, so to get back to a dosage of .3mg/l:
(.381) * (.3mg/L) * (5 gallons) = .6ml of cupramine to add to the tank.
This is how I handle dosing copper. Also, if you don't already, make sure you get a copper test kit and check the copper levels and adjust as necessary.
Let me know if you have any further questions.