There are three fish ponds I am in charge of that must stay running no matter the weather. Sometimes the hardest part is just being able to get to them on snow covered roads.
This is what deep cold looks like on the upper koi pond. As long as the water is still flowing, the ice falls don't matter.
Koi and goldfish are totally cold hardy. As long as the pond does not freeze solid they will be fine. They go into a hibernation stage and stop eating as the water cools down.
The lower koi pond was a solid sheet of ice when I arrived. It also has a leak that is exacerbated by the deep cold. The water level was way low. I added water and starting ice breaking as the sheet began to melt.
There ya go fishies. All better now, until it freezes over again in this weekend's deep cold.