Thorny

Sorry to hear of Your loss. I think You should install a good size air pump. One air stone on a pond that big ,in My opinion is not enough. I only have a a 2000 gallon setup. On that I have 135 liters of air going in a minute. It goes to two 9 inch air diffusers, and a 12 inch long AIr stone. I also have a trickle tower with 7000 litres of water passing through it an hour, and the water returns to the pond by falling 5 inch`s so loads of air there aswell. If You dont like The water disturbance you can always stick your airstones in Your filter chambers.
Get on Ebay You will pick up a have tidy pump for sensible money.
Then hopefully Your problems will be behind You. You get so many benefits from air, Better bacterial growth, better Koi Growth, better water quality and more important better fish health.
Justin