In my own experience, I battled ich for about 2 years. I initially treated all my fish in a hospital tank (copper for 2-3 week) then just let the copper level drop after that naturally, with water changes until putting the fish back in the DT, while leaving my tank fallow for the recommended 72 days. Each time I would do this, within days of returning fish to the DT, spots would reappear. I would tear the tank down, catch the fish, threat with copper in a hospital tank, and let the tank go fallow for increasingly longer amounts of time, over 90 days at a time. Each time, within days of putting fish back in the DT, ich would return. So it seemed to me like even the 72 day number was an underestimation. The last time I let the tank go fallow for 3 months, I also ran 2 - 40w Lifegard UV sterilizers. And after doing that, my fish have not experienced a recurrence of ich in over a year now.
So I don’t know what to think about that 72 day number. It seems like not enough time to me. My fish in the hospital tank showed no signs of ich until I returned them to my DT, and then just about all fish were re-infected within days. So it always appeared like my DT was never fully cleared. So I still recommend waiting longer than 72 days to anyone that runs fallow.
But that was just my experience...
