Koi Fish in Winter

There are Necessary Steps for Keeping Your Outdoor Pond Survivable for Koi Fish During Colder Weather

As a rule, cold winter months and the resulting decrease in water temperature can pose a unique challenge when it comes to proper care for your koi fish. It is crucial to initiate koi pond winterization measures so the pond water can support the survival of the fish during cold weather.

My backyard pond in winter - the heater is hard to see, but it does create a small hole to allow gas exchange

A completely frozen pond can deplete much needed oxygen levels and increase the likelihood for the buildup of toxic gases, such as Carbon Dioxide (CO2) or Methane. Therefore, in climate zones where the temperature range can include harsh winters, a number of essential details are necessary, especially regarding maintaining adequate water quality with a top notch filtration equipment.

Your koi pond should try to be at least 4-5 feet deep if you plan on leaving the koi fish outside throughout the colder months which would give them an adequate distance to move away from the frozen surface. Clean oxygen levels tend to be more plentiful at the bottom of the pond which is where the fish tend to gravitate during the winter.

When the water temperature falls below 50 degrees Fahrenheit, the metabolism of the koi decreases, minimizing their movements and their desire and necessity to eat...in effect, putting them into a semi-hibernation stage, also known as torpor, that will last until the warmer months arrive once again. 

When your pond temperature dips below 50 degrees Fahrenheit, it is time to stop feeding and start thinking about winterizing

What Happens to Koi Fish When Their Body Temperatures Decrease

When frigid temperatures come back around in the coldest months of winter, depending on the climate zone, many changes will happen internally to the koi fish in your backyard pond, specifically affecting their metabolism, immune system and bodily functions.

It is important to reiterate that these pond fish actually do not require regular feedings when the water temperature drops below 50 degrees Fahrenheit. This is due to a significant decrease in their metabolism and activity level in order to preserve their energy. Koi fish will typically spend most of their time at the bottom of the pond, where the oxygen level is a bit higher, with very minimal movement. 

While it is beneficial in some respects for your koi to develop a slower metabolism leading to energy conservation, it can pose a health risk when their immune system also slows down during this period. As spring arrives and brings back warmer temperatures, there is a degree of harmful bacteria that can enter the body and cause various health issues. Therefore, a decrease in immune system function during the winter puts koi fish at a considerable risk for illness once the temperature swings back towards summer. This is something that is crucial for a koi keeper to be aware of and keep a watchful eye.

Koi fish can certainly thrive in cold water temperatures during the dark winter months with proper winter pond maintenance procedures in place, however it could be deadly for them if the water temperature dips below 35 degrees Fahrenheit. 

Your koi will enter a state called "torpor" where their metabolism slows down when the pond water gets cold

Mother Nature sure does provide these resilient fish with some spectacular adaptation methods to help them survive until the arrival of warm weather.  A reduction in their bodily functions - notably breathing, metabolism, blood flow, digestion of food and their immune system - all contribute to the hibernation mode or half-asleep state they maintain throughout the winter. 

Some Helpful Tips for Winterizing Your Pond for Cold Temperatures

There are many pivotal elements to winterizing a koi pond in order to ensure the health and well-being - and overall survival - of your koi fish. Here are a few suggestions which should typically be done in the fall before the water temperature decreases significantly:

  • Pond Netting to keep leaves and other harmful debris from entering and settling at the bottom of the pond. Leaves can clog pipes, drains, and other parts of your pond system.
  • Pond Thermometer to indicate when to stop feeding your koi fish (when water temperature decreases to below 50 degrees). Pond thermometers are inexpensive, easy to install and allow you to monitor the water temperature in your pond 24/7.
  • Cutting back aquatic plants and water lilies to 2 inches above water level and 1 inch above their crown, respectively.
  • Pond Heater and De-Icer to provide a way to maintain an open ventilation hole in the ice for the necessary exchange of gases - the penetration of essential oxygen and the release of poisonous gases. The very lives of the koi fish will depend on this crucial detail, as without it, they will not survive.
  • Introducing beneficial bacteria, also known as cold water bacteria, to your pond will go a long way to increasing the quality of the water and maintaining the proper pH balance. It also proves to be most helpful with the reduction of fish waste and other particles, such as leftover food.
  • Consider a pond cover to provide valuable protection of the fish during harsh weather by warming the water during sunny days and trapping that heat throughout the night. It also provides a buffer from extreme winds and prevents ice crystal formation on the water.  Easy to assemble and disassemble, a pond cover can also be used year after year.
Koi pond netting is available from a variety of retailers and can help to keep your koi pond clean (and predators at bay)

When is it Unsafe to Keep Koi Fish in an Outside Pond During the Winter

The most important thing to keep in mind when it comes to the survivability of your koi fish once colder weather is upon us is that they cannot survive if the pond is frozen over.  However, freezing to death is not the only thing to avoid, but also the stress that can be placed on your koi as the warm water cools, which could in turn lead to illness on a body that will already be immuno-compromised when colder weather starts. 

While there are advantages to bringing your koi fish indoors during the winter, some things to consider when determining your risk to benefit ratio are factors like:

  • Could extended cold snaps reduce the ability of the outdoor pond heater/de-icer to successfully maintain the necessary hole in the ice for gas exchange? If your pond is below 2 1/2 feet deep then it will potentially be at a higher risk for freezing over.
  • Keeping in mind that koi fish continually grow during the winter months, how many and how big are the fish that you currently have and would there be enough space in an indoor pond or aquarium? Healthy water quality to support bigger fish would require higher water volume.
  • Relocating koi fish from an outdoor pond to an indoor environment could add additional stress to an already vulnerable fish.
  • Does your indoor aquarium equipment have the proper filtration system to dispose of the additional waste that would be deposited? The metabolism of your koi fish would increase due to warmer temperatures indoors.

Safely Transporting Koi Fish Indoors for the Winter

If you have determined that it is most advantageous to bring your koi fish inside during the cold winter months and the indoor tank is properly set up, preferably with water from your koi pond, transportation to the new temporary winter home can occur.  The goal here is to transport them in the least stressful way possible into a healthy environment. 

Depending on the constraints of your pond, and your climate zone, it is possible you may have to relocate koi during the winter

The recommended way to bring your koi fish inside is to place them in plastic bags (or containers). Gently place the bags on top of the water so the fish can acclimatize to the temperature change prior to being released. An important thing to note here is that koi fish are very talented "jumpers", especially in their first few week in a new environment. Therefore, a cover is a highly suggested way to avoid any unintended dangers.

Be sure to consistently add beneficial bacteria and check the water quality and pH in the tank to prevent ammonia and nitrate levels from potentially reaching toxicity. It is also recommended to replace 10-20% of the water every 1-2 weeks, more frequently if there are high levels of ammonia or nitrate.

And...Back to the Outdoor Koi Pond

When the warmer temperature arrives in late spring, a move back to the koi pond should be an easier proposition using the same method. Before doing so, however, be sure to clean the pond and remove any debris that may have collected during the winter. The pond should also be up and running, with a good pump and filtration system, for at least a week before the koi are brought back.

As the spring arrives and the water temperature warms, you'll be able to resume feeding

The koi fish can easily develop a parasitic reaction if they are placed in water that is inconsistent with where they have been acclimatized. Water temperature in the outdoor koi pond should never be more than 10 degrees cooler than the tank from which they came.

Koi Fish Are Very Adaptable

Known to be a very sturdy and robust fish, this beautiful member of the carp family has natural ways of successfully adapting to cold weather environments. Nature provides them with resourceful methods to counteract the effects of the harshness that comes with winter living. With an overall reduction in their bodily functions, leading to a state of semi-hibernation, koi fish essentially nap their way through winter and rejuvenate with the arrival of warmer water and springtime temperatures. Winter koi pond maintenance will determine their health and well-being, however, they are known to survive severe winter conditions fairly well.

Frank Salvatore

In 2015 our family moved into a house south of Denver, Colorado with a  koi pond.  Since that time I've learned to really enjoy the koi fish and the pond. This blog is dedicated to providing helpful hints and information for koi pond hobbyists - as well as those of you who just inherited a koi pond and are thinking NOW WHAT?

About Me

Hey there - I'm Frank Salvatore. In 2015 our family moved into a house south of Denver, Colorado that had a Koi pond. The problem was I knew absolutely NOTHING about koi ponds.

This blog is dedicated to providing helpful hints and information for koi pond hobbyists - as well as those of you who just inherited a koi pond and are thinking NOW WHAT?
Learn More About Me

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