Simply Koi and More Koi

I would like to open up a discussion on when do we treat or not. We have all had that odd koi when it has stopped eating but is still swimming around but not with the other koi. Also when nothing is showing on several skin scrapes.( including gill biopsy) How long do we wait and see what developes. What sort of time frame should we wait if the koi in question does not get any worse in the hope the koi will overcome any problems? Does this make a difference if the koi is adult and female.Also what if the problems are internal parasite? In such a case is it best to isolate the koi and treat or would it be better to treat the koi in the pond with the other koi even though the other koi are showing no signs of disease?