By Bob Passovoy, President MPKS
First: standard disclaimer. We live near Chicago, in Zone 6. If you live in milder or colder zones, adjust accordingly.
Only the most fortunate of us Pond People have the luxury of a year-round season. The rest of us have to put up with Ma Nature at her most unpleasant, otherwise known as winter. Our fish will shut down most of their nonessential systems, including their gut and their immune systems, at water temperatures approaching 45 degrees Fahrenheit, and our bio converters are shutting down as well. At the same time, any trees and plants around the pond are attempting to dump a ton of biomass in the form of dead leaves.
Our primary job in this case is to begin early in the fall and prevent the eventual problems this combination of events will produce
First: Cleanout. This is by far the most important thing you will do to protect your fish, especially if your plan is to leave them in the pond during the winter. (This is necessary for all but the smallest of your Koi. Koi larger than a couple of inches will quickly overpower any standard aquarium filtration system. Any fish brought in for the winter will need an established large-capacity bio converter and plenty of water.)
While the air and water are still warm, get into the pond and remove all of the season's sludge and debris. This will minimize the generation of breakdown gases (primarily hydrogen sulfide) during the months when your pumps are shut down, your water is not moving, and areas of low oxygen content can develop in deeper parts of your pond.
If you have goldfish, you will be pleased to know that the comets, shibunkins, sarassas and other long-bodied types are hardy in this area. Round-bodied fancy golds like lion heads, ranchus, ryukins, are marginal. We take ours in, as our marsh garden is somewhat exposed. Other folks leave theirs out, but we can't recommend it; our winters have been too unpredictable lately.
Second: Leaf Netting. Once clean, the pond should be protected with netting fine enough to keep leaves out. The netting needs to be supported by a framework high enough to keep it several feet above the water's surface (to keep fish from becoming entangled) and strong enough to support not only the net, but also the plastic sheeting you're going to put over it as air temperatures drop below 40 degrees at night. I've found that the best solution to this is a greenhouse or "poly-house" kit, usually formed from pre-bent 1 ¼ inch galvanized pipe. Kits of this type can be had cheaply from Midwest Trading in St. Charles, IL.
Third: Know when to stop feeding. Fish are cold-blooded. When the water temp drops, the fishes' digestion stops. Food will ferment in the gut and kill them. In cool weather, it may take three days to completely digest a meal, so you need to be reasonably sure the water temperature will stay up for the next 72 hours. They will still beg when it's sunny, but don't give in. Fall and spring food should be low-protein - easier to digest - Cheerios will do nicely.
Fourth: Ice Management. You'll need to keep at least some of your water surface ice-free during the coldest months to allow for gas exchange. Without protection from wind, even the most robust trough heater will be overwhelmed by a really cold Chicago-area winter, leaving you the unenviable task of huddling out by the pond in subzero weather melting your way through several inches of ice with pots of boiling water*. What fun! An air stone set just below the surface of the water and running "wide open" will help, but nothing beats protection from the wind. Use 7-mil plastic over the pond. This gives a strong greenhouse effect on sunny days, blocks the wind, and keeps the pond still and quiet so the fish are calm.
There are a number of high-tech solutions to frigid water, mostly in the form of pond heaters of various designs. Electric heaters , especially trough heaters meant for farm use, are energy hogs, usually not up to the job, and rust out quickly, often creating short circuits in your pond and injuring your fish**. More elaborate systems involving heating coils filled with a heat-exchange fluid and heated by a modified hot water heater (usually powered by natural gas) can be expensive to install and run. The coils require a basin of circulating water to keep water temperatures up or need to be put into the pond itself. This may require considerable redesign of your pond at a tough time of year.
A flashier and much more compact system based on the instant water heaters becoming popular in newer homes allows reverse exchange flow through a heat exchanger and can be hooked directly into a pond's main return. It can also be run off propane. More information can be obtained by contacting Steve Morelli at Keirin Koi, who is a friend of the inventor.
In any case with water heaters, your target temperature at the bottom of the pond is 39 - 42 degrees. The fish need the rest they have evolved to expect. If you have a January thaw with bright sunny days, you may need to turn the heater off. At 50 degrees, parasites and harmful bacteria become active. Fish won't have an immune system until the water hits 60 degrees. Also, if the fish wake up enough to move a lot, they are burning their fat faster. This is not good if we continue to have long, cold springs.
For those of us on the "low-tech" (see "cheapskate" in your Funk & Wagnall's) end of things, if you have set up your weather cover and secured a layer of 7 mil poly-house covering over it, simply shoving a $30 Home depot oil-filled radiator under the cover in a secure and level area and adjusting the heat level to a setting that keeps the air temperature just above freezing will do the job just as well.
Continuing to run your water system at winter temperatures will "super cool" the water, disturb and eliminate the warmer water stratum at the bottom, and overstress your fish, resulting in a very high mortality rate come spring. The less you mess with the pond in the winter, the better your fish will do. As the water temperature approaches 40 degrees, turn off your pumps and filters, and either drain them or protect them from freezing. Frozen water expands significantly and will explode your piping. Pull your air stones up off the bottom and leave one or two running just below the water's surface.
As the water cools, water at around 39 degrees will settle to the bottom of your pond. Your fish will congregate there, since it'll be the warmest part of the pond. Koi handle temperatures down to 39 degrees without undue stress, and will stooge around near the bottom, nibbling a little algae all winter and living off stored fat. The less they are disturbed, the less fat they will burn and the better their condition will be in the spring.
* Don't break the ice! Water is not compressible. The shockwave can kill or injure fish.
** Electrical current through water causes permanently bent backbones in fish.