Sorry to hear about your Koi. Do you have a reputable koi dealer in your area ? Maybe a koi or watergarden club in the area ? I'm trying to think someone in those area that would have access to a microscope.
My first thought is some kind of fungal or microbial infection.
If you could wetslide the slime coat on the infected fish and rule out parasites-- it might tip the scales to that of fungal or microbial.
I hope someone can chime in with some experience to help.
I've seen fish waste away rather quickly with fungal rot that on parts of the body go to angry ulcers.
try KOIPHEN.com they have a fish emergency section with SUPER GURUS when it comes to fish care.
good luck !!!!
Hope the remaining herd pull through !
