The Fish Keeping & Aquarium Guide.

Can Koi Fish Survive Winter in Outdoor Pond?

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All that koi need to last in a frozen pond through the winter is enough oxygen in the water. Koi are cold-blooded animals, usually referred to as poikilotherms.

In simpler terms, the water surrounding a koi’s habitat affects the body temperature of the koi. As the water temperature changes, so too do the body of the fish and its functions. All reactions occurring in their body depends on the temperature of the surrounding water. When it’s winter and the pond is frozen, koi will reduce their biochemical reactions to a halt.

In this situation, you usually can find the koi in the pond’s warmer regions, which is at the bottom, often swimming. If they don’t swim and move around but stay still all winter, their joints will become stiff.

When the water temperature falls below 50⁰F, the metabolism of the koi also diminished significantly. Koi will enter a conservative state known as torpor to cope with this temperature. Torpor is a minor hibernation where reduces all their reactions and activities. In this state, animals can survive extended periods with little or no food. The energy that would usually use for higher activities is thus, conserved. Extended torpor periods hurt koi and therefore are ill-advised. In vast cold stretches, the immune system of koi will likely be weakened or permanently shut down.

If koi are in torpor, you would need to stop feeding them after a specific time. It is due to the enzymes that aid in the digestion of food in koi fish are usually absent in cold periods. Thus, the food will not be digested and stuck in the koi gut, which may cause health issues in the winter.

How to Take Care of Your Koi in the Winter?

If your pond has a depth of at least four feet deep, you can allow your koi to remain during the winter provided you consider these options:

1) Clear All Leaves Present In The Pond Before Winter, Whether Tree Or Plant-Based

As the pond temperature drops, ensure you clear out all waste from trees, leaves, and other contaminants before the water reaches 15⁰C. If you don’t do this, the pond’s oxygen levels will be low through the winter months. The leaves and other organic matter will deteriorate during the winter months and increase the ammonium and nitrate levels of the water. In simpler terms, the water quality will be bad.

In addition to this, you will need to excavate the bottom of the pond, removing any sludge or debris present. Removal of all organic matter from the pond is of great importance because any organic matter that did not remove will decompose with time, use up oxygen, and release harmful bacteria into the water, thus reducing the water quality. Even though koi can tolerate low temperatures, they still require clean water to function.

If you want an easy way to keep your pond clean, you can prevent organic matter from entering the pond by covering it in the fall. A pond netting will help you achieve this. A fine mesh net or pond vacuum will help to remove any organic matter or sludge at the bottom.

2) Pond Treatment

Koi fish are often susceptible to external factors under lower temperatures. Check both your koi and the pond are free of parasites before winter to protect them from any infections and diseases during the winter.

Winter isn’t the most favorable weather for pond fish. When the water is cool, the fish’s immune system is weaker and less effective against infection. It also takes longer for fish to heal due to slower metabolism. The wintertime also affects diseases, slowing them down. However, this does not stop their action against fish. Pathogens and bacteria won’t spread as quickly as they usually do. Instead, they will take a longer time to work.

If the fish has not prepared properly, then their immune system will be suppressed by the cold. When temperatures fluctuate or are low, then it can be an indication of a disease. Even though the seasons occur naturally and unable to control, a pond keeper can maintain and protect fish from the effects.

Usually, pond keepers apply anti-parasite treatments to their ponds in the early winter or autumn to prevent infections. This method is best suitable for warm temperatures and may be ineffective in cold climates. The decision to use this treatment in warmer temperatures does not depend on the safety of the fish. Instead, it is based on the treatment potency as it is less effective in colder climates. The treatment will help to reduce the activities of parasites that can harm the fish. At this period, the fish’s immune system is not stable, so the fish is vulnerable. The treatment helps protect them.

You can also use medicated food, feeding them to the fish to help ward off bacteria. While you can still feed your koi before winter, it is a good idea to provide your koi medicated feed again. It is worth noting that this should complete before the temperature reaches 55–65 °F.

3) Maintaining a Hole in the Ice

You will need to aerate your pond should you choose to let your koi spend the winter outdoors. You can achieve this by making an opening in the ice. In uncontrolled environments, koi lakes don’t usually freeze completely. It is because there is some part of the water that doesn’t get cold. The lakes are not in regular shape lakes, they have banks that slope slightly, and the salinity of the water varies greatly.

On its own, ice isn’t a danger to koi, provided that the pond will not freeze totally due to the pond being deep (more than 4 feet). The only harm to the fish is when the concentration of gases becomes toxic and diminishes oxygen due to the sealed over water.

If there is no hole in the ice, then the fish won’t survive the winter. Creating an opening in the ice allows useful gases in and toxic gases out. If you need a way to create and maintain an opening in the pond, you can achieve this with a pond heater or de-icer.

You can also maintain the hole by using an aerator. The water will not freeze because of the water currents that arise from rising bubbles. It also aids circulation, facilitates the exchange of gases, increases oxygen levels amongst others, to prevent the water from being harmful to the fish. You can improve the chances of success by using de-icers/pond heaters in unison.

You should not smash the ice in a pond. If you do, you create shock waves that travel through the water to the pond’s lower areas where the fish may be at rest. It can end up causing discomfort and hurt the fish.

Given that the freezing of the pond doesn’t go all the way to the bottom, and there is a hole at the top, then the fish will live through the winter.

4) Heating the Pond

You can also get your koi fish through the winter in a pond by using a heating system. The heating system’s main components are a boiler, a heat exchanger, and a temperature probe. The heating system will allow your pond to maintain temperatures higher than 12°C by heating the water.

Some of the uses of a water heating system include

  • Maintaining summer temperature ranges in the pond through the year.
  • Avoids freezing in the pond.
  • Preventing temperature fluctuations that occur in ponds during spring.
  • Stopping the large temperature swings that occur in a pond in the spring.
  • Aids the koi in crossing “Aeromonas Alley” quickly by heating the water.
  • It also improves the Koi’s immune system, which aids the fish in case it’s sick.

In addition to heating the pond, you should filter clean the pond, continue feeding the fish and monitor the water changes. It will ensure the water pH doesn’t crash.

5) Using Cold Water Beneficial Pond Bacteria

You can boost the pond’s water quality by using cold water bacteria, bacteria designed to thrive in cold temperatures. It can also create a biological balance in the pond by adding it at the beginning of spring.
You can also minimize fish waste and particles during winter by using cold water bacteria. It will thus prevent an accumulation of toxic gas in the water.

The cold weather doesn’t stop the action of bacteria. Instead, it only slows it down. So, having a large bacteria population can improve the natural exchange process in the winter. If you introduce a de-icer and an oxygen pump to the water, the bacteria will continue working, usually even when the weather is coldest.

Through the autumn, you can slowly supplement the pond to improve the presence of helpful bacteria in your filter as well as on the pond floor. It can also prove useful in boosting the ecosystems of the pond and the bacterial population by the time spring comes around. It makes them a great addition to the pond.

6) Build a Greenhouse to Cover the Pond

Greenhouses have a function to protect ponds from harsh winter temperatures. It prevents the pond from the influence of cold winds, which draw heat from water bodies. Building a greenhouse isn’t tricky, and it can be used and reused yearly. It will ensure that ice doesn’t form on the pond.

You can manually create your greenhouse with materials like PVC or wood with plastic sheets. You can also buy an already made greenhouse kit and place it around your pond in the winter. With this, you need not be bothered about your pond freezing.

This method allows you the option of regularly checking in on your koi. Should you install a heater, you can maintain a stable koi population all year round without interruption.

Spread a plastic sheet across the frame of your pond, and hold it in place by putting heavy objects on the edges of the frame. You can try this or any of the other easy methods of making a greenhouse around your pond. You can get creative to keep your pond warm. It is important to remember the importance of a koi heater.


There are still some factors that could cause your pond to freeze over with all things in place. Power failure, lousy equipment, maintenance neglect, amongst others, must be avoided at all costs. The main aim of caring for your pond in the winter is to ensure you get an excellent start to the following season. So, you should ensure that all your actions and care create an environment favorable to the koi having a solid immune system.

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