Copper and Qtank that will be converted into Refugium

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Have a drilled 20g that I'm eventually going to hook up between display and sump as a refugium. I'm also looking for a Powder Blue Tang so thinking the 20g probably not a bad size for a QT first.

Still reading copper may not be the best for a tang with ich, in case it develops, and hypo-salinity might be better choice, but if I do use copper, could the tank later be tied into a reef system?