An important thing to consider when you are designing a new pond is how you will over-winter the fish. In the Upper Midwest and other areas that have freezing temperatures for an extended period of time, it is critical to understand what is needed for Koi to make it through the winter. To learn how to over-winter Koi in cold areas, you must first understand the concept of temperature stratification of water.
If you plan on running a water feature in a pond during freezing temperatures, it is best to design the pond so that there is a deep cut-off section that is out of the main flow. If it is too late for that, then the next best thing is to re-position your water pump so that it is up off the bottom right by where the waterfall dumps back into the pond. This will recirculate the cold water and leave the rest of the pond relatively isolated.When you jump in a lake in the summertime, the warmest water is at the surface, and it gets colder as you go to the bottom. In the winter, it is the exact opposite. The warmest water is actually at the bottom. This is because water is most dense at about 39 Fahrenheit, so as water cools, if undisturbed, the 39F water drops to the bottom and colder water stratifies above it. This is why we have ice at the surface of the pond instead of at the bottom. Therefore, it is critical that when water temperatures begin to drop, circulation of the pond water is either stopped or modified to allow thermal stratification of the water to occur in the deepest portion of the pond.
Also, if you aerate the pond to keep a breathing hole open, it is critical that the air stone or diffuser is not placed in the deepest portion. Rather, it should be off to the side of the pond in an area that is only about 50% of the maximum depth. If you are not running your water pump through the winter it will be necessary to install an Aeration System or De-Icer. FYI, aeration systems are much more energy efficient than water pumps and de-icers, and they experience fewer problems.
In areas that have freezing temperatures for an extended period of time, it is best to have at least 3 feet of depth if you intend on keeping the Koi in the pond through the winter. Goldfish can usually get by with at least 2 feet of depth. If, for some reason, you are unable to achieve these depths, you may want to consider installing a Pond Heater or bringing your fish indoors for the winter.
Finally, it is critical that Koi begin winter in very good condition and that they are left undisturbed. Since they do not feed through the winter, they must survive off the energy that they have stored up. Think of it this way, they must go into winter with a full tank of gas. Every time they are disturbed, they burn some of that fuel. If they run out of gas before they are able to refill the tank in the spring, they are likely to succumb to one of many ailments. Check out the Koi & Goldfish Nutrition page for info on “filling their tanks” before winter. To keep from disturbing Koi, you should give them a hiding spot in the deepest portion of the pond away from moving water. We use large plastic culvert pipe on our farm, or many people build a cave with flagstone, etc. Finally, you should ensure that this location is not near a high human, or animal-traffic area. Every time Koi are startled, they burn fuel. If they do this too often, they will not make it through the winter, or begin spring in a weakened and vulnerable state.