Today my ranchu pond has reached the low of 6 degrees C (42 F) that I hoped for. All fish look well. A couple, almost a little too well! Although moving more slowly, two of the ranchu still cruise about for much of the daylight hours, apparently grazing on the bottom. The others are much more sedate, hovering near the bottom and moving less frequently.
I hope this is perhaps just a sign of the strength of certain fish and that they are not struggling or uncomfortable with the concept of the resting period. I also hope it will not create issues when attempting to trigger them into spawning behavior in later weeks.
I may drop the temperature a little further and see if this helps force all the ranchu to rest.