No offence, and I hate to sound like a know it all, but I believe Coolshades first post where he says "because you will start to get temperature inversion, which is not a good thing" is the most accurate.
Temperature inversions mean still, stagnant, and often, low oxygen, water on the bottom of the pond.
Fish are cold blooded, and winter tolerant fish like koi and goldfish can easily take very cold water as long as certain other needs are met, and high on that list of needs is adequate oxygen levels. Of course if they are going to survive cold winter waters they also need to be free of dangerous build ups of ammonia, carbon dioxide and other harmful dissolved gases as well, and the best way to do that is provide a way for those gasses to get in and out (or prevent them from occurring in the first place), that's why ponders provide holes in the ice allow for gas exchange. A common misnomer is that these holes are simply there to allow toxic gases (like hydrogen sulfide, H2S ) out, in reality it is a gas exchange that is necessary, this means oxygen in, as well as well as those other gases out. But a hole by itself will not be nearly as effective unless the water moves past it in some way, and that means it is better to circulate the water somehow, either by a aerator or a pump.
I also believe, and this is where my view differ from many other ponders, that you should circulate the water at the bottom of the pond as well.
Unless you circulate the water at the bottom of the pond it is difficult for the oxygen to reach those lower levels adequately. High oxygen levels at the bottom pond are important, not only for the fish to breath, but high oxygen levels help prevent the build up of H2S in the first place, because the microorganisms that produce H2S can only survive and do their dirty work in organic mater rich, and relatively oxygen free environments.
Here is some info taken right off wikipedia about H2S, "Sulfate-reducing bacteria obtain energy by oxidizing organic matter or hydrogen with sulfates, producing H2S. These microorganisms are prevalent in low-oxygen environments, such as in swamps and standing waters. Sulfur-reducing bacteria (such as Salmonella) and some archaea obtain their energy by oxidizing organic matter or hydrogen with elemental sulfur, also producing H2S. Other anaerobic bacteria liberate hydrogen sulfide when they digest sulfur-containing amino acids, for instance during the decay of organic matter. H2S-producing bacteria also operate in the human colon, and the odor of flatulence is largely due to trace amounts of the gas. Such bacterial action in the mouth may contribute to bad breath."
In all but the smallest ponds it is rare for fish to get so cold in the winter that they can't survive. Only ponds where the ice gets so thick they run out of free water underneath the ice layer. It is however, very common for fish to die in the winter, even in large ponds or small lakes, from lack of oxygen.
Read the info on the link below about winter fish kills, to see how it happens and how to prevent it.
Winter fish kill explained.My 2 cents
