Mourning the loss of Ninja

It is with sadness that I have lost my most beautiful butterfly koi to a critically low oxygen level in my pond. I was told by Creative Water Gardens, my sole source of pond fish that I need to immediately drain half the water and refill, adding dechlorinator which I luckily had on hand. I have never had a situation like this in the 7 years that I have had my pond. It is horrible that I did not know that oxygen levels could be depleted to a level that fish would actually suffocate, the larger fish effected first. I had seen him gulping bubbles at the waterfall edge; that was his effort to get more oxygen, but I did not recognize the sign. In the picture, Ninja is the bright orange fish with black spots.