Sudden Koi death

Question
For reference, I have four outdoor connecting ponds, with the bottom pond around 8,500 gallons, total ponds, 10,000 gallons.  Filtered with Bio-Falls and skimmer.

Eric, I lost one medium size Koi last year and one smaller one a few days ago.  Both of these fish were under a year old.  Both fish showed good health and vigor and the latest one to die, made it through a mildly harsh winter.

He was very active, eating well, swimming like an Olympic athlete.  No signs of any health problems.  He ate hardly as usual one morning; by the next morning he had separated himself from the group and was just lying on the bottom quietly.  He stayed that way for five days, not eating, not showing any signs of disease and poof, he was dead.

He was a long fin fancy Koi, we called Finnies. When we fished him out of the pond, his body was naturally covered with some Algae and still no signs of any disease.  This is exactly how we lost another Koi last year.

Do you have any idea what may cause this sudden, slow death?  All other Koi's and Comets look healthly and are doing well.  Our lower pond is a little over four feet deep in the center and as I said, holds around 8,500 gallons.  The pump exchanges 10,000 gallons per hour.  We clean our skimmer bag every few days.

The Comets have started spawning, the Koi not quite yet.  Except for adding a little yearly pond salt to the pond, we do nothing extra to the pond, except clear dead leaves in the Spring; (that escape our netting system) healthly plant life, Salamanders, frogs and an occasional Rat snake that likes to eat the frogs and lots of birds that take baths in the upper ponds.

The bottom pond is protected by the water "predator" and a Blue Heron Decoy to keep our little darlings safe.

Any ideas what may have killed those two, otherwise extremely healthly Koi?  Water clear, all levels checked for amonnia, ph, etc., all normal.

Thanks so much, Eric.

Heather in Virginia

Answer
Hey Heather, your pond sounds impressive and you are diffidently on top of it. I think you could be a pond expert! I can't tell you anything you don't already know. I have had fish all my life and sometimes they just die for no reason. All I can say is remember if you want to breed keep your fish population down to a minimum. Most people I talk to have to many fish in there pond and when they breed there is more and the weakest will die first, but they will die. I have salt water fish and I keep good care of them but from time to time they just die for no reason. I have bought fish at the pet store that got sick and killed the rest of my fish. If you buy fish anywhere make sure you watch them for a couple of weeks to make sure they are healthy before you buy. Once my ponds or aquariums are set I don't buy any more fish. Hope this helps Eric