You can do a freshwater dip at any time during the QT process. If you want to do it to confirm flukes are present or because the tang is having difficulty breathing, then do it now. It's not really something you need to do to every new fish. If you are going to treat with prazi, and velvet is not suspected, then just skip it and go ahead and treat with the prazi.
If you can, complete the 30 days of copper, remove copper, then do the two doses of prazi 5-7 days apart. You can safely combine prazi with copper treatment if you need to do so, just be sure to have plenty of O2 in the tank using an air stone or small powerhead.
