Weve come up with a whole heap of answers here and on your original post from Toxic algae to salt copper manganese etc however non of us have come up with difinutive proof of what is causing it whether this is throwing us from the answer or not I dont know .
Has your pond guy taken a scrape of your koi to see if this is parasitic , it may be a bacterial problem or last but not least a viral problem.
A big killer of koi virus wise is a virus known as KHV or koi hearpes virus which can be brought on by stressful situations like overcrowding high water temperatures etc if it its it would kill around 80% of your stock however youd need to send a fish to a lab to see if this is the case , which would mean you sacrificing one.
KHV symptoms can be distressing they show signs of severe gill damage a loss of colour , produce exessive amounts of mucus and severe internal problems where the organs liquify but your not discribing any of those symptoms .
It is temerature related at 7c it is dormant at 27c active .
However I doubt very much if you have it Tricodena can cause losses as can whitespot if untreated in reality we need a scrape done and a prasite count of that scrape under a microscope cn your pond guy do this for you Brenda ?
One thing I dont understand about him is this if he has 25 years experiance with koi , then why on earth didnt he warn you about your stocking levels being far to many for your sytem to continue to support ?
Dave
