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.
If you don’t want to deal with the outdoors during winter you can set up a tank indoors. Most koi 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.
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, have appropriate depth so that fish can get to warmer areas of the water, and ensure the fish don’t become stressed, so they'll be healthy come spring.
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 is not dangerous in itself, 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 build up. 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 the 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.
Be careful not to overfeed koi prior to and during winter. Koi will stop eating once the temperature gets below 50°F. They live off the fat their bodies have put on in the late spring and summer. Their movement will slow down, along with their metabolism and they will in turn need less oxygen. When the temperature reaches 50°F, Koi should only be eating wheat germ foods, as the bacteria in a fish's digestive system are no longer able to process food efficiently. Once the water temperature falls below 40°F, you should stop feeding your fish.
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. The needed depth of the area depends on the minimum temperature in your zone, and can range from 18" to 9 feet deep. 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. As a general rule, 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 is one of the most important parts of the winter routine. 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 and a lot of movement of the water there causes them 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. Finally, DO NOT use a hammer or other instrument to crack a hole in the ice as the vibration will also disturb the fish.
Follow these guidelines and your Koi will stay happy and healthy all winter long.