Hello William,
Here in southern Manitoba it is impractical in my opinion to keep the pond open during the winter without resorting to costly & risky methods. Unless the pond is very deep (4' plus) the chances are minimal of keeping it open during those stretches of -35C weather even with a bubbler and heater. It would not take much of a power interruption or heater failure to have things freeze over solid under those conditions. I bring my little beasties indoors for the winter to a homemade wintering tank. It is basically a 2' x 4' x 16" deep wooden box (about 60 gallons) lined with surplus pond liner on a stand to raise it off the floor. I use an external cannister filter & some plants to keep the water fresh along with a 2 x 4' bulb fluorescent light. My lilies actually bloomed last December! I've attached some pics of my setup.
My recommendation is to save yourself from some needless anxiety during those stormy, cold days ahead and bring your fish indoors for the winter. Plus you get to enjoy them for the winter months. Pull the pump & filter out and leave the pond filled to freeze solid. Otherwise the liner will likely collapse in when the ground begins to thaw and the spring runoff and rain find it's way under the liner.
