I lost the only two koi I had this winter, and a large goldie. This was our second winter in this house, and the second winter we had the pond. It was installed by a previous owner, without any filtration, and I've mostly been doing this by the seat of my pants for the past year.
The previous winter wasn't as cold, apparently. As I recall, the ice got only a couple of inches thick. I have a 100 W deicer, and it kept a hole open all winter. This year, the deicer worked, too, but the ice layer got to be about 9-10 inches thick.
In addition, the fish all grew enormously last year, since I don't think they were fed regularly by the previous owner. And the goldies had lots of babies. so the fish load was probably higher, and may have resulted in toxicity in the water, although the day I removed the dead koi, I stuck a test strip in the water, and nitrates, nitrites looked fine.
This summer, the pond gets as much of a remake as I can manage without taking out the liner and starting over. Maybe next year. Everywhere I look, I see violations of the good pond-building rules I see here and other sites.
Esther, I feel your pain.