Koi Pond Winterization

“Fish pond in winter” by Muffet is licensed under CC BY 2.0

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A well-stocked koi pond is quite an investment. That’s why when the temperature drops, proper koi pond winterization is so important.

Fall Preparation

In order to ensure the well-being of your koi and the integrity of your pond, you will need to begin in the fall. First and foremost, this means cleaning your pond of any debris. Organic matter breaks down over time and will reduce water quality and oxygen levels over the course of winter. Installing a pond net over the pond prior to fall will help in preventing leaves and other organic matter from accumulating in your pond. 

Koi Pond Filter in backyard koi pond

Use a skimmer net for floating debris and a pond vacuum for everything else. You should also take this time to trim and prune any aquatic plants.

It should also be mentioned that not all pond liners hold up the same over repeated winter conditions. Pre-formed PVC liners hold up the worst in winter conditions while RPE liners hold up the best. For more info check out this article on how various liners hold up in winter conditions

Preventative Amendments & Practices

Another consideration are your koi’s health going into winter. While most harmful bacteria and parasites slow down in cold temperatures, so does a koi’s immune system. A minor problem can turn into a full-blown disaster come spring when the water warms back up. “An ounce of prevention is worth a pound of cure” as the saying goes. 

There are a variety of amendments and practices that will prepare your koi for winter. These include:

  • Add Beneficial Bacteria – These cold water bacteria serve to break down any remaining organic material like leaves and sludge in the pond. This helps to maintain good water quality through the winter and into springtime. 
  • Feed Cold Water Food – Coldwater koi foods are specially formulated to be easily digestible. The primary ingredient added for digestibility is wheat germ. The best koi foods will also have things like spirulina and beneficial bacteria. These feeds should generally be fed when the temperature is between 55-65° F. 
  • Treat For Parasites & Disease – As stated previously, a preexisting health problem can turn into a full blow emergency by spring. That’s why it’s a good idea to treat for parasites and bacteria when the temperature is between 50-55° F. The products I would recommend for this are Aqua Meds Prazi and API Pond MELAFIX

Fall And Winter Feeding

As I previously stated, feeding should stop completely when the temperature drops below 50°F. But even before the temperature drops, this low feeding needs to be reduced based upon the water temp. 

Koi Water Temperature Feeding Frequency
Water TemperatureFeeding Frequency
73-80° F
23-27° C
4-5 / Day
66-72° F
19-22° C
3 / Day
61-65° F
16-18° C
2 / Day
56-60° F
13-16° C
1 / Day
50-55° F
10-13° C
1 / Week
<50° F
<10° C
Do Not Feed


Do Koi fish Hibernate?

Yes, koi fish hibernate in a sense, and this state is called “torpor”. During the summer months, a koi’s metabolism and activity are at very high levels. But as the water temperature drops so does their metabolism. This is because koi, like all fish, are cold-blooded.  

This torpor state usually occurs when the water temperature drops below 50° F. At this point the koi’s biological functions like digestion, metabolism, and immune system slow to a crawl. This is why you need to stop feeding your koi at these temperatures. They will likely reject any food offered during this stage anyway. Any food that your koi does eat will not be digested properly. This can result in health issues for your koi.

Koi Pond Winterization Options

Koi fish are a hardy species and can live in very cold water. As long as the water doesn’t freeze solid and they have adequate dissolved oxygen they will be just fine.  

Many people successfully keep koi outdoors during the winter even with most of the pond iced over. Alternatively, you have the option to house them indoors if you choose.

There are pros and cons of each method, and which is best, depends upon your specific situation.  

1. Move Koi Indoors

If you have the option, moving your koi to an indoor koi pond or holding tank is a good option. This way you can ensure they make it through the winter. With your koi indoors, you will have the opportunity to keep a close eye on them. Your koi will also continue to grow provided you keep the water warm enough. 

The downside to this option is often lack of space. Fully grown koi are very large and need a lot of room. Depending on how many koi you have and their size, housing them indoors may not be an option. 

If you do go this route, stock tanks used for livestock are an affordable option. These can be found in capacities of up to about 400 gallons and only cost a few hundred dollars. If you have a garage this would be an ideal place for your tank. Don’t forget that you will still need a water filter and possibly a heater if your space isn’t climate controlled. 

Pond Equipment Winterization

If you decide to house your koi indoors during the winter you can either shut the pond down entirely or keep it running. Just make sure to drain and clean any water pumps, filters, and water lines not in use. 

If you do decide to keep the pond running you will still need to pay attention to water levels and watch for any ice jams.

2. Keep Your Koi In The Pond (No Cover)

Most people opt to keep their koi in the outdoor pond during the winter months. Koi will be just fine under a layer of ice provided they have the proper setup.

Pond Depth

First and foremost, you need a pond depth suited to your climate. Depending upon where you live your pond needs to be at least 3 – 5 feet deep. The deeper the better if you live in a very cold climate. For example,  a 2-foot pond will simply not do in the frigid Minnesota winter. 

A deep pond provides a few different benefits. The most obvious is to prevent the pond water from completely freezing. In northern climates, one foot or more of ice may form. In these areas, an 18-inch deep pond may vary well completely freeze. Even if it doesn’t completely freeze, the koi will have very little room below the ice.

A deep pond also has the advantage of absorbing thermal energy from the earth. While the outside air may be well below freezing the ground below the frost level will be comparatively warm. This warmth will be transferred to the pond, keeping the water temperature above the ambient air temp.

Pond Equipment Winterization

Warmer climates may leave water pumps, filters, and features running throughout the winter. If there is the possibility of freezing, these should be adequately insulated or shut off and drained. It is generally OK to shut off the filtration system during the winter months because the fish are producing very little waste in their torpor state. 

If there is a chance of freezing you will need to install an air pump with a bubbler. While koi are fine under ice they need a small hole somewhere in the ice. An air stone will serve to oxygenate the pond as well as keep a hole in the ice for gas exchange.  The ideal depth to install this bubbler is 10-12 inches below the water surface. At this depth, it will not disturb the warmer water below. 

         **Tip – if the water does completely freeze over do not try to smash through the ice. Fish are sensitive to shock waves and vibrations. Instead, if the ice is thin enough place a pot with boiling water on it. For thick ice, you can use an electric drill.

You may also need a floating de-icer to maintain a hole in the ice in extremely low temps. 

Waterfalls need some extra consideration. While they may not freeze over in some climates they can negatively affect the water temperature. Waterfalls circulate the water in such a way that it comes into constant contact with the air. 

This means that during warm days the waterfall will rapidly warm the water and vice versa when the temperature drops. This temperature fluctuation will stress the koi and may cause health problems. 

3. Keep Your Koi In The Pond (With Cover)

If you want to give your fish some extra protection in their outdoor pond you can cover it. There are a variety of ways to do this, but they all serve the same purpose; to retain heat and prevent freezing. 

Your pond cover can be as simple as laying some pipes or lumber across the pond and covering it with a solar cover. Or, my favorite, you can build a greenhouse over the top of your pond. This way, you can enjoy your koi pond year-round. 

Whichever cover option you choose, consider the potential snow load. Can your cover hold the weight of the snow? You may need to promptly clean off the cover after each snowfall if the structural integrity is suspect.

Pond Equipment Winterization

Depending upon how warm your pond stays you may want to keep filters, pumps, and water features running. Unlike an uncovered koi pond, a covered pond will likely remain unfrozen. 

You can even operate a water heater without running up your electric bill too much. This way your koi will be active and growing year-round. If your water temperature stays above 50° F you will need to continue to feed to fuel this growth. This also means that your fish will be producing waste and the pond will need its filtration system running. 


I hope this gives you some insight and options to consider for your koi fish this winter. However you choose to overwinter your koi pond plan and prepare early.