I have often wondered how someone maintains a koi pond thoughout a Minnesota winter. I have sold a few homes with ponds and waterfalls that hold these beautiful fish that resemble a giant goldfish. I did sell one in the winter and the buyers were koi-experts. They knew if the pond wasn't properly winterized, the fish could all be dead under the ice. Heck...I didn't think they could survive in the pond!
So my curiosity got the best of me today and I did some research. I found this article written by Mickel Mcsuche that explains the process of Maintaining Your Koi Pond During the Winter. Enjoy!
Clean Up- Take about a weekend to completely go over your pond. Clean up and unwanted bulk material in and around your pond. Inside your pond, clean up all leaves, slit, and other material from the bottom. Also, remove any plants or flowers that will not make it through the winter. Around the pond, clean up anything that can blow into your pond, as you are not likely to notice this debris until the end of winter. Taking the time to make sure that as much debris is removed as possible will prevent potentially harmful bacteria and parasites in the future.
Stop Feeding- You must remember to stop feeding your Koi during the winter. When fall begins and the temperatures hover around 55 to 60 degrees, only feed once a day. Once the temperatures drop below 50 degree for the first time, stop feeding completely. Even if the temperature goes above 50 degrees, still reframe from feeding your fish. The majority of Koi, when healthy and the temperature is above 50 degrees, take at least four days to completely digest food. If you mistakenly feed your fish to late the food will not digest and will end up killing your fish. Do not mistake you Koi as hungry when they open to their mouths to you. This is more of a learned reflex then hunger. If you are concerned about not feeding them, remember that fish eat other things besides the food you provide them, especially if your pond contains a large amount of natural plant life. If they are at all hungry and you are not feeding them, they will fill up on this.
Try to do seasonal check up on your koi pond and all of the equipment like your filter and your medication. Since the majority of ponds in the world lie dormant during the Winter, you are less likely to be able to find the products you need.
You also need to prepare for the cold weather by purchasing all the items that you would need during the summer. Koi have been known to withstand constant temperatures as low 39 degrees, and temperatures slightly lower then 39 degrees, for short periods of time. You also need to make sure that your heater is big enough to heat your pond when you are buying one, because if it is not able to heat the whole pond, ice will form which can caused trouble by the gas trapped in the ice. In extreme events, it may be a good idea to have an emergency tank inside available.
Turn Off All Water Sources- In colder temperatures, your heater will be working hard to maintain a water temperature suitable enough to keep your fish alive. If you have water features such as waterfalls, streams, or constant moving fountains, make sure to turn them off during Winter. This is because, this water feature can circulate more cold water into the pond, So, your heater only need to heat the water in the pond without having to do extra work to heat up the circulation of cold water.