Thanks Dave, here in Medicine Hat, we get some of the coldest weather on planet Earth! (Zone 2) We get minus 45c (and sometimes colder) and that's not the wind chill either. In the middle of winter we get what's called a Chinook, it can go from minus 45 to plus 5 overnight, and then we all get crazy and it's like "ya ho! HEATWAVE Bring out the pool, get the shorts on!!! ha ha ha
I winter the fish with a big 1500watt cattle trough heater, and I still get lots of ice on the three ponds that circulate together, but under the ice the water still flows. The bottom pond can get a foot of ice in some areas, I can walk over top of the ice and see the fish! The bottom pond gets the most ice, so I have the small goldfish there. The top and middle pond can appear quite froze too, but I keep the pumps running from pond to pond and the water ways freeze right over top, but under the ice, just like a river or creek, the water still flows. Been wintering the fish for 20 years this way, nobody in the pond seems sick or unhappy... the sick and week just sort of disapear.... from time to time. (Someone comes and takes them away to the hospital where they all make a fast recovery) (ya right, that's what happens, sure...)
Really, the fish do very well UNDER the ice but not IN the ICE! Some of my fish are over 20 years old now.