What copper treatment is that?
Copper is typically maintained for a 14 day period (some prefer 21) and then it can be removed and the fish allowed to remin in the
QT for an additional 2-3 weeks to monitor for re-occurances or secondary infection.
Hyposalinity should be maintained at treatment level (14-16
ppt) for a minimum of 4 weeks and then the same after treatment time/precaution. The fish can actually survive hyposalinity levels for a few months without issue.
Choose copper (preferabley Cupramine) or hyposalinity for treating
C. irritans,
never both.
IMO, treating prophylactically is not a good idea. If the fish comes down with something completely different, the current treatment must be removed and the new one started. The lost time can usually mean the difference between success and failure.
Cheers
Steve