Can Koi Fish Survive Winter?

Koi are hardy fish, but they do have some requirements when it comes to the cold. They are dependent on their surroundings for their body temperature and don’t have their own internal heating system like mammals. In other words, they are poikilothermic. 

Koi can survive the winter under certain conditions. There must be airholes in pond ice, the pond must be at least four feet deep, the water must not freeze solid, they must be fed the right food as the weather cools, the pond must be free of organic debris, and the water quality must be excellent.

If the pond freezes solid in winter, they can die. They will stop eating if the temperature drops below fifty degrees Fahrenheit. If they have accumulated enough fat during the summer and late spring, their bodies will use it for nutrition, but their metabolism slows right down, and they barely move.

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What Happens To Koi When The Water Gets Cold?

Koi Eventually Stop Eating

The cold slows down koi’s metabolism and interferes with their ability to digest food. They, therefore, fast when temperatures drop below a certain level. This is normal and nothing to worry about, but you have to reduce their amount of food to keep them healthy. When they enter a state of torpor, they shouldn’t eat at all. 

At temperatures between fifty-nine and sixty-eight degrees Fahrenheit, you should only feed them twice a day. You should also only feed during the warmest part of the day. Depending on how willing they are to eat, you can even reduce feedings to once a day in this temperature range. 

Always be aware of their behavior and how they respond to food, as they will generally only eat if they are hungry.

When the water temperature drops below fifty-nine degrees Fahrenheit, you have to switch to a wheat-germ-based food. Give them small quantities at a time to gauge their feeding response. If they seem disinterested in food, don’t feed them as it will just turn to waste in the water. 

You need to stop feeding them completely when it falls below forty-one degrees Fahrenheit, so it’s essential to have a thermometer for your fish pond. 


Koi Become Less Active

Koi are most active at temperatures between sixty-eight and eighty-six degrees Fahrenheit. This is when they consume the most amount of food. When the temperature drops below sixty-five degrees Fahrenheit, you will notice a change in your koi’s behavior. They are less interested in food and swim more slowly. 

At temperatures between forty-one and fifty-nine degrees Fahrenheit, you will need to switch to a different, less protein-rich food because although they will still eat summer food, their bodies can’t digest it properly. This means there will be more fish waste in the water. 

To avoid this and still give them a chance to build up enough energy reserves for the winter, you need to feed a good quality, cold water food. In the depths of winter, they don’t need any food because the bacteria in their gut go dormant. 

Weakened Immune System

Their immune system also becomes weaker, making them more vulnerable to bacterial infection by harmful pond bacteria called Aeromonas or Pseudomonas. The temperature range between forty-two and sixty-two degrees Fahrenheit is known among fish keepers as Aeromonas Alley. It causes fin rot, mouth rot, and ulcers in koi.

When the pond water temperature is sixty degrees Fahrenheit, the koi’s immune system operates at less than fifty percent. Unfortunately, the bacteria are resistant to the cold and still active. Aeromonas Alley isn’t confined to the winter months. 

It is also a problem in spring as the water slowly heats up because the fish have fasted all winter, and their immune systems are weak. They are therefore vulnerable to attack by parasites and bacteria. It is impossible to rid a pond of Aeromonas bacteria as they are endemic and part of the typical pond environment.

The bacteria usually feed on fish waste and decaying matter in pondwater. They live alongside the fish, on their bodies and even in their intestines. This isn’t a problem when the immune system is fully operational as it keeps sickness at bay. Heat also helps control the Aeromonas, but when temperatures drop, they can multiply.

Ulcer Disease


Aeromonas hydrophilia, also called ulcer disease, will slowly but eventually kill koi. The symptoms of both Aeromonas and Pseudomonas infections are the same – skin ulcers that grow big enough to expose muscle tissue. This leads to sepsis that spreads into the fish’s bloodstream, causing its kidneys to fail. From there, matters get worse as the koi can no longer osmoregulate, and complete organ failure ensues.

Due to the abuse of antibiotics by koi keepers in the past, Aeromonas is highly resistant to them. Even if the fish recovers, its beautiful pattern will be marred by scarring. Prevention is, therefore, better than cure.

Koi Enter A State of Torpor

When the koi get cold, they enter a state of torpor, similar to hibernation, and hang almost motionless in the water. If it is too shallow, their undersides may rub against the bottom of the pond leading to ulcers and skin lesions. This opens the way for Aeromonas and other parasites to take over.

Insufficient Oxygen In The Water


If the pond ices over, this can inhibit gaseous exchange between the water and the surrounding air, reducing the amount of available oxygen. It also seals in harmful gases like carbon dioxide. The koi will quickly use up the remaining oxygen in the pond water and start struggling to breathe. If there is insufficient oxygen, they will suffocate and die.

How To Help Koi Survive The Winter

Apart from the feeding regime already mentioned, there are several ways to help koi survive the winter. If you don’t have the space to transfer your koi into a temporary indoor tank during the winter, you can still keep them alive outside in the pond.

Proper Pond Design

You can save yourself a lot of headaches by ensuring that your koi pond is designed to overwinter your fish. The sides of the pond should be smooth, vertical, and without any undulations at the edges. Features such as islands, peninsulas, and channels should be avoided because they create dead zones where oxygen cannot freely circulate in the water.

The pond should be a minimum depth of four feet to help the fish survive freezing winter conditions. The water at the bottom of a pond stays warmer than the top layer because it isn’t in contact with the air, and the fish can take refuge there. Ideally, a four-foot-deep pond should have a length of eight feet to maintain its water quality and ecological balance.

If you want your koi to become veritable giants, however, the pond should be between six and eight feet deep and twice as long, but the costs of filtration and oxygenation will be higher. Koi will seldom venture deeper than seven or eight feet. Installing a heating system in a shallower, smaller pond is also cheaper.

In milder climates, you can get away with a pond that is three feet deep, but koi get their exercise by moving up and down in vertical water columns, not horizontally. They can also be more stressed in shallow water and accidentally flip onto the bank.

De-Ice The Koi Pond

If you come out in the morning to find your koi pond has iced over, you need to break an air hole in the ice. You do this by placing a metal pot or kettle full of boiling water on the ice and melting a hole gradually. 

Do not hit the ice to break it, as this will cause shock waves in the water that stress the fish. In their already weakened state, the shock could even kill them. Koi naturally require a cold period in their seasonal cycle in which to rest and are evolved to withstand winter conditions.

Koi have an organ called the lateral line that is sensitive to changes in water movement and pressure. If you hammer the ice with an ax or other heavy tool, the water pressure this creates could damage the lateral line, and the koi will die.

They acclimate as the temperature of the water decreases with the seasons and their bodies naturally prepare for winter. However, they shouldn’t be subjected to sudden and dramatic temperature changes.

Prevent Ice From Forming

It is not harmful to a pond to have some ice over it as long as there is a hole that allows the gaseous exchange to take place. You can install a pond heater that floats on water and keeps an air hole open for the winter. These gadgets are also referred to as pond de-icers.

There are other energy-efficient alternatives to keep the water from icing over. You can build a greenhouse over the pond that collects the sun’s heat or in which a heater can be installed.

Keep an air pump going to promote gaseous exchange and prevent the pond from freezing solid and killing the koi. Ponds freeze from top to bottom, and most won’t turn to solid ice because the top layer prevents the deeper water from direct contact with the cold air. A pond is more likely to freeze solid if it is small and less than four feet deep.

Don’t Overstock Your Koi Pond

One koi needs at least two hundred and fifty gallons of water. A standard three-foot pond holds a minimum of one thousand gallons which means you can only keep four koi in it. If your pond is overpopulated, the stressful winter conditions combined with potentially lower oxygen levels may cause the spread of disease and kill them.

You should always keep pond water as clean as possible at all times to ensure healthy koi. Good water quality is essential. However, you need to be particularly diligent in the fall and winter when plants shed a lot of dead material and the wind blows it into the water.

Keep The Pond And Surrounding Area Clean At All Times


Remove all leaf litter twigs, branches, and other dead plant material from the pond and its surroundings. Clean the bottom of the pond using a long-handled pond net, making sure to scoop up as much sludge as possible. Decaying plant matter can adversely affect the oxygen levels in the pond during winter.

Empty the leaf skimmer every couple of days in the fall, and if there are any trees and shrubs overhanging the pond, trim them away from the edges. Remove any tropical pond plants from the pond, including water lilies, and throw them in the compost. If you have hardy water lilies that can survive the cold, move them to the deepest part of the pond after trimming them back two inches above the crown.

Install A Pond Net At The Beginning of Fall

At the beginning of the fall, install a pond net to stop leaves and other debris from falling into the pond. When it is full of leaves, roll it up and dispose of them. You may have to do this repeatedly if you have a lot of trees and shrubs in the pond’s vicinity.

Add Beneficial Bacteria To The Pond Water

Adding sufficient beneficial cold water bacteria to the pond will break down the nitrates and ammonia in fish waste, improve water quality and keep it clear. They will also digest sludge and pond scum and are available either in liquid or powder form.

These bacteria are available from koi pond specialists. Always buy genuine products from reputable koi dealers. Do not buy generic products from hardware or garden stores as these may be for septic tanks, do not contain the right bacteria for your pond, and may have contaminants.

Using beneficial bacteria can save you considerable work when spring comes around. The powdered form should be scattered in the middle of the pond, around the pump intake for your biological filter, and in front of the skimmer. You should also sprinkle it around the pond’s edges.

Shut Down Pumps, Waterfalls, And Filters In Winter

If temperatures remain consistently below freezing in your area in the winter, it’s better to remove the water pump and filter media from the pond. Clean them and store them indoors. 

You should still use a small air pump to keep the water oxygenated and keep a hole in the ice. You may also need to use a de-icer in frigid temperatures.

Agitating the water too much in winter with water fountains and the like could stress the koi when they are in torpor. You can keep the pond running in a milder climate, but an air pump and airstone or aerator may be necessary to keep surface ice open in the odd cold snap. You may also have to top up the pond occasionally if there is surface evaporation.

A waterfall can disturb the warmer water layers at the bottom of the pond where the koi take refuge in the winter, causing them to drop in temperature. There is also the risk that a waterfall could drain the pond if the remaining water in it freezes, so it isn’t the best way to keep the pond from freezing.


Install A Heating System

You could install a pond heating system that consists of a temperature probe, heat exchanger, and boiler. This will keep the water at summer temperatures in the winter. However, you will then need to feed your koi in winter and do all the filter cleaning and water changes you do in summer to keep the pH at the right level.

Some people say that koi need a seasonal cycle to keep them healthy and that winter ponds can be beneficial. The fish need to slow down their metabolisms and rest. You may thus wish to seek expert advice for your particular pond before installing a pond heating system.


If you are an attentive owner and keep your koi healthy, they should survive the winter provided you follow the steps we have outlined. You need to take account of your local weather conditions and the size, depth, and design features of your pond to decide which equipment will work best. However, keeping the pond clear and clean of debris, ensuring sufficient water oxygenation, and good water quality is critical. 

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