Water change after copper treatment

[zilbermm]

Hi,

I have yellow Zebrasoma and a Cardinal Banggai in a quarantine tank and have just finished two weeks of Seachem cupramine (preventive treatment). I want to start treating with Praziquantel and Metronidazole (pure powder) now before i move them to the main tank but first i understand i need to perform a water change.
My questions -
What percentage of water should i change now and how many of those should i do?
How do i treat with the Praziquantel and Metronidazole and for how long?

The tank capacity is 60 liters, running with a protein skimmer and external filtration.

Thanks in advance.