No clue...but, I'd add salt, to put a slime coat on all other fish. It will also kill simple bacterias and fungi,so if anything is lurking, it may get it under control. Are you using a an enzyme "good bacteria" pond additive, such as KoiZyme?
I can't imagine gill flukes would have that quick of an effect, with no other "pre" symptoms such as gasping for air around the water out-take. It's also odd for only one to die when all pond conditions seem to be ok, and no other fish are showing signs of anything.
I jumpstarted mine with Zyme. Looked it over and the skin was perfect. Put in another tank with oxygen and salt. but it was too far gone
Just kep watching the others. They are behaving just fine and eating every chance they get .
No no new fish. Lost another one yesterday I guess. But found it in the skimmer. Had been there a while going by the smell. LOL
The rest of them are doning fine.
Lost my waterpump today. Have to go to town tomarrow and get another one. Away goes another 200.00
Oh My! Sorry about losing a second koi. Do you know why it got into the skimmer? Losing a pump is a bummer. It's happend to me too. I now have a spare, completely plumbed and ready to drop in at the first sign of trouble. My pump went out in the middle of a Saturday night. No way to get one until Monday and it was touch and go keeping the water oxygenated until I could get the replacement. I kept draining off some old water and adding fresh with as much splash as I could arrange. All day and all night Sunday. I don't ever want to have to do that again. What brand pump was it. Mine was a Danner Mag Drive. The impeller wore out.
What water tests do you use? If it is a dip stick I would get some API dropper tests because they are more accurate. With such a high ph just a little amonnia could be toxic and bleeding from the gills could be a sign.
evesta - You've go it! pH 9 is high (and the outer limit of tolerability for koi). I don't use the testing strips for either of my ponds or my aquarium. They're just too unreliable. Ammonia is much more toxic at a high pH. You're one smart cookie to catch that.
Checked the PH and it is still high. The water source is OK, but rain runoff makes it go back to a high number. Cannot do a large water change because it will go below the skimmer intake. Have a bottom drain so it is easy to let out a little every day. I added one pint of muriatic acid yesterday and another today. Will check it again in the morning. 4500 gallons is a lot ot time to refill.
I admire your efforts T5. I don't have bottom drain. When I do a 50% water change ( which I just did) the fish just hang around in the ever shrinking water. It does go below the skimmer but I've turned off the pump anyway so it doesn't matter. I have a separate pump just for water changes that I lower into the deep part with a veeerrry long hose that pumps the water out into the lawn. I do this twice a year. However, I'm only dealing with half the volumn you are so it doesn't take as long. I use two hoses to put fresh water back in so it goes pretty quick. The fish love the refill part. A twelve hundred gallon water change takes about 2 hrs hours total. If I vacuum the bottom of course it takes longer. Rain here is acid/soft and actually lowers the pH. All the surfaces surrounding the pond slope slightly away so that there is no ground runoff. Geography sure makes a difference doesn't it? Is your pond 9000 gal? That's huge!
If you don't have the UV's installed in the Savio skimmer start a piggy bank, save up, and install them. The bulbs do have to be replaced after 12 months of use time on the bulb. It's still worth the cost. You'll have no regrets.
One 25 on each. That provides more bulb surface area and a greater volume of water passing in front of the bulb. You want as much water exposure to the bulb as you can get. More exposure means better algae kill.