A chelated copper like Copper Power or Coppersafe are much easier on most fish than "ionic" coppers like Cupramine.
That being said, I've found Cupramine to work just fine as long as you "do not follow the label directions." The directions have you do two big doses to reach therapeutic. This can be pretty tough on some fish. If on the other hand you slowly dose Cupramine over 4-5 days to reach therapeutic, it allows the fish time to adjust to the copper and gives you time to observe for the "copper sensitive" fish before it is too late and the fish doesn't make it through the treatment.
Measure your QT accurately so you know exactly how many gallons, including the filter.
Multiply the number of gallons by 4 (drops of Cupramine) = total number of drops to get to therapeutic levels.
So an actual measured 20 gallon QT x 4 = 80 drops total. Spread the dosing out over 4-5 days equally. And if possible, not just one dose per day but AM and PM.
Use a Hanna HL Copper Checker to measure the copper as you are dosing. Do so more frequently as you get closer to the calculated number of total drops of Cupramine. You may find you actually will use a few less drops or a few more. Avoid the color matching test kits. They are horrible, and over/under dosing is way too easy to do.