OK, just re-read the whole thread, maybe I need to breakdown and get myself eye glasses, cause I am not sure where I got the 150 koi from, old age must be setting in for me!
I see that prior to the fish dying off, there was a "algae bloom", and that would indicate to me that the water temperture was getting warm and an excess of amount of nutrients must have been present to have a "rare winter algae bloom".
Then came the sudden cold that I must have saw on your temps a while back, a few nights I saw that your night time temps were very low, like minius 20 Celsius, which is very cold for your region. I don't think your ground froze hard by the sound of it, and with a deep pond like that, the earth down deep will keep the pond warm, unlike here, where my water remains ice cold as my ground is still froze hard down deep. So the warmer water, and excess nutrients created a "rare spring algae bloom", and gases most likely came with the algae bloom, then came your extreme cold and the surface of the water may have froze over, and/or the hole in the ice was not sufficent enough to let all the gases escape. I myself have never seen a algae bloom in the "dead" of winter, in an ice cold freezing pond up here, cause our water would not get that warm with the ground frozen rock hard for miles and miles.... Here up north, even though we have had some warmer temps now, the ground is still very frozen solid, and so my water is still extremely cold, so things in pond that may be dead are not rotting yet as the water is still too cold. that is why fish can survive in our ponds that freeze over much harder, and not have a rotting mess or a strange algae bloom in the middle of the winter, and then too the extreme cold water causes the fish to breathe much less as well, using much less oxygen.
Then there is the stinking rotten issue of SNAIL's! Trapdoor snails and large snails do die off sometimes. When a big snail dies, they hide down deep, under things, and I made the mistake years ago of having large Apple snails for a few years, plus I did also have trapdoor snails too. I can tell you that if I missed getting any of those big snails out of the pond before the water got ice cold, they would die and in the early spring when the water temps started to get warm, I would find their dead bodies inside the shell, all gooey and gross, and the rotten smell was enough to make one gag instantly! I since gave up on large snails, and I tried the trap door snails and found they rarely survive the extreme cold here and won't risk having them die off down deep and create a stinking rotten foul cloud of gas. Now trap door snails normally would probably do well in your climate, but this winter was a record cold breaker for many people this year, and I must admit I was very surprised to see that you had such extreme cold this year for your region. I think the extra cold that you had early in the winter, could have killed of a few big snails.
The extra deep pond can be a big issue itself, as such a deep pond can hide lots of waste and rotting decaying life, and most nets do not reach down that deep and then you probably do not have a real good view of what is down deep hiding too, So I can only conclude that the warmer earth kept the water warmer, which led a "rare spring algae bloom", and the deep pond hid much waste. Then came the extreme cold and that iced off much of the pond, combined with the heavy snow fall, and the pond was "smothered in gas".
