Winter is coming, and this will be the first Winter that you go through with your Koi pond. Think of Winter as a down period for your pond, as less events happen during Winter then any other time. However, there are special precautions that you need to take before Winter arrives, to ensure that your pond and fish survive.
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.
Check Up- Do you seasonal check up on all your equipment. This includes everything from your filtration system to your store of preventable medications. 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. Make sure your emergency kit is ready and up to date, which should include medications, bags, nets, and your water testing kits.
Prepare For Cold Weather- Prepare for cold weather by investing in the items you will 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. When buying a heater, remember to research what size you will need to adequately heat your pond during the winter, otherwise ice will still form, causing potentially dangerous amounts of gas in the water, due to it being trapped under 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. These features will circulate water, and constantly bring new, and cold water into your pond. With these switched off, the only water that your heater will be responsible for is the standing water in your pond.