Keeping Koi in Your Pond During Cold Months

Koi add a beautiful element to any pond but can they survive the brutal winter? Good news! You can keep koi in your pond during cold months with a little work and some warm clothes. 

Koi fish are cold blooded and can stay alive in cold temperatures. Their metabolism relies on the temperature of the water that they live in. During winter, when the water in your pond is ice cold, the koi's metabolism slows down to a crawl. They spend most of their time treading water at the bottom of the pond, while sometimes swimming around a bit so that their joints don’t seize up. 

The key to successfully keeping your koi healthy over the winter in any climate involves a few vital actions. Your pond must have enough oxygen and gas exchange, you must feed your fish correctly for the temperature, and your pond must have appropriate depth so that fish can get to warmer areas of the water. 

The most common reason fish die over winter is due to a lack of oxygen.  

A good way to prevent this is to test your water regularly to see the oxygen levels. Ice itself is not dangerous, the danger to fish comes with the reduced oxygen and increased toxic gases caused by the sealed over water surface. Having an aerator running in your pond during these cold months can keep the water moving and promote the exchange of gases. An aerator is a lot more economical than a pond heater, and it will oxygenate the water while reducing ice buildup. Having a small hole will provide ventilation if you’re aerating the water. A deicer will keep at least part of the water above freezing and create a small hole. The hole in the ice will allow for gas exchange since the koi will need fresh oxygen and the carbon dioxide that they exhale will need to vent out. If the carbon dioxide level in the water rises to high levels, a buildup of carbonic acid will occur, and a pH crash will turn the water acidic. Once that happens, your pond will be too toxic to support aquatic life. Keeping winter-hardy plants, like bog plants or non-tropical water lilies, inside the pond can produce oxygen and use up carbon dioxide. DO NOT use a hammer or other instrument to crack a hole in the ice as the vibration will also disturb the fish. 

Be careful not to overfeed koi prior to and during winter. 

Fish are less active in cooler water, which means they need less food than they do in the warmer seasons. Fish will generally slow feeding when water temperatures are at or below 60 degrees. When the water temperature is steadily between 55-60° F, only feed once a day and switch the food to a wheat germ-based diet because these foods are highly digestible at lower temperatures. When the temperatures stay between 50-55° F, reduce feeding to once a week. If the temperature stays below 50° F, you can stop feeding them until the weather warms in spring. It is important not to overfeed because uneaten food will add excess organic matter to the pond increasing the chances of low oxygen levels and a fish kill. Discard any remaining fish food since the nutrients in any open packages will disintegrate over time. Sometimes there are warm spells during the winter seasons and fish may come to the surface. Be sure not to feed them as they may be coming up for air and any food they eat will not be digested. 

Fish must have a place to find refuge when the water in the pond freezes.  

Water reaches its densest point at 39.16°F. In the wintertime, the warmest water is at the bottom of the pond. That is why koi spend most of their time there.  Knowing the frost line in your zone is important. You can call your local garden shop, extension service, or building inspectors to find out where the frost line in your zone is. Above the frost line, the temperature gets colder than 32°F, and below, it will generally stay above freezing. Generally, the deepest area of the pond should be at least double the depth of the frost line, just to be safe.  

Keeping your fish stress free  

Stress causes immune systems to falter, affecting your koi’s health. Some ways to keep your koi stress free are to not overstock your pond, so the fish don't have to compete for space, and minimize water movement where the fish are. The fish will be resting at the bottom of the pond. If there is water movement, it causes the fish to stress because it requires them to use vital energy to keep their bodies stable in the moving water. If you use an aerator or any other kind of equipment in your pond over the winter, use lower GPH to ensure that only the top part of the water is moving. Treat your pond with a broad range parasite treatment before the water temperature falls below 50°F. Any fish containing parasites over the winter will be untreatable due to the lower water temperatures. 


Alternative methods 

If you don’t want to deal with the outdoors during winter, you can set up a tank indoors. Most fish winter tanks are 100 to 1,000 gallons in size as the fish need space in the tank. If you are fine leaving the fish where they are, you can also install a pond heating system made up of a boiler, heat exchanger, and a temperature probe. But before you install anything, you must first establish the groundwork for healthy fish by preparing your pond in the fall, including doing an aggressive clean-out of the pond. 


Follow these guidelines and your Koi will stay happy and healthy all winter long.