I would run it as a completely separate system to avoid possible re-contamination, don't use water or rock/etc from your display. I would dip, then place into quarantine for observation, if they continue to decline I would consider medication, but I do not know what to recommend for that.
I feel that if there is a parasite in the display and you dip then return to the display you could be reintroducing them to a contaminated environment and that may explain why you are only seeing a slowdown of their recession.