I am going to quote Richard Heimberger. He is a pretty smart guy and is a professional pond keeper for some folks that have very nice ponds in the Chicago area. I got a chance to meet him on the recent pond tour I attended. He addresses aeration of ponds during the winter in his latest newsletter:
"An odd subject for August, I know, but I got tired of hearing how everybody lost their fish due to the bad winter. First thing you must realize is that winter for a fish is the same every year in USDA Zone 5. They are cold (33F) and they are in the dark. Regardless of below zero wind chills, fish stay at 33F. So what was the problem? I couldn’t keep a hole in the ice with my heater. No wonder your fish died. If you think a heater, by itself, will keep a hole in the ice...you're wrong. Besides, two square feet of open water will not gas off the carbonic acid in 33F water enough to save your fish. Everybody I talked to did not have adequate aeration, or had no aeration at all, to gas off CO2. That is why your fish died. Aeration is mandatory in Zone 5 to keep fish alive during winter. You must have a minimum of 45 lpm per 2,000 gallons of pond water. Aquarium size aeration is inadequate for pond use. Even if a dome of ice forms over where the bubbles are coming up, you are still gassing off CO2 under the ice. Melt a hole in the dome with a watering can of water, and it will vent the ice.
My fish endured 33F for 90 days, and they are still alive. I have 9 fish, each about 60cm, in 2K gallons of water. Why is this? The answer is simple. I had aeration. "