Sick Koi Gradual Losses
Koi sometimes suffer various illnesses and diseases which don't kill many of them all at once. But that doesn't mean it can't "go that way" in a short period of time. I highly recommend that you get a complete grip on your ponds' water quality and then proceed with some sort of diagnostic process or perhaps a shotgun remedy that would intercept most conditions.
It might help to refine your koi's conditions by visiting the "symptoms" page of this web site.