Ian & Terri,
thanx for your post and pics.
I can relate to your problems. Winters are usually not that cold over a prolonged period of time overhere, but they are long and it takes a long time in our climate to heat up the water the natural way in spring. However, sometimes it becomes pretty cold for a few days or a week (last march -21 or -22 c, without the windchill).
The first year I used my present pond (1998), the temperature dropped very quickly. Having just moved, I did not have a clue how the water temperature would react, but since it is 2m deep, didn't worry to much. That night I came home, and when I walked by the pond, I saw a 55cm Kujaku lying on it's side. scared me to death. Measured the temperature, and it was _0.7c. The pond should have frozen over. But because of the strong winds, and unfavorable direction, and hardly any shelter because there were no plants in the garden yet, it did not freeze. It supercooled! I saved the koi in a desperate effort by adding warm water slowly. But they sure weren't happy campers. Lesson learned

Have been keeping the pond covered after that, and now run a heated winter pond.
So don't know exactly when crystallisation starts, but this was to low.
Regenmeneer