Lost my thermometer last month under the snow, and back then the surface water temperature was 31 degrees, not much difference with the bottom temperture, and I don't think it has warmed up, rather it is much colder now. When I took a piece of hornwort out of the pond last week with one inch of water in dish, water was starting to freeze before I even got to house with it two short minutes later. I can tell you it is minus 20c at least right now and if I unplug the pond heater the open pools of surface water will start to close before my eyes! Yes it is cold, water does not get much colder, large portions of connecting ponds are froze over. Fish are alway fine, even the fancy little goldfish with big fan tails and the koi always make it every year just fine, matter of fact I rarely lose any fish. I think the last time I lost fish was a couple years ago big blue heron eat some before I fixed it so he never could come back with big strings high above pond from trees to house. Have not lost a fish to winter kill for many years now, and my pond does get very cold water, but that water runs all year like the river or creek and it does very well for me. My koi are mostly solid colors but I do have some fancy koi as well. Everybody fine here in Canada, my method has worked well for over 22 years so I don't panic anymore or worry.
If you ask me the biggest problem with winter is too many fish crowded under the solid ice and they run out of fresh air supply and gas build up can cause way more problems than ice cold water. Don't forget if you are heating your pond with big heater like me, you can loose lots of water due to evaporation when temps extremely cold. Don't forget to top off your pond in winter too!
