Wow- I didn't think they could survive under the snow and ice! I guess it was the water quality then...
Amazingly my pond still gets a lot of leaves & branchs from the Willow trees that surround it and my filtration is pretty lacking. I'm hoping this year to have a new filtration set-up to handle any water quality issues I may have.
Once November hits, I only run my waterfall pump (which usually keeps a hole in the ice as the ice forms around the waterfall)
As well I throw in a cattle trough heater as well to ensure that there is a hole in the ice.
Now the pond is roughly 5000 gallons and I only have 19/20 fish in it (17 Koi and 2 or 3 goldfish) so that probably helps in that it isn't overpopulated.
There have been a few of us Manitoba ponders who have started keeping our fish outside during our "rough" winters and interestingly the fish are doing pretty well considering the temps.