Can Koi Fish Survive the Winter?

Koi pond in winter ice

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Koi fish are amazingly hardy animals and can tolerate a wide range of temperatures. In their native waters, koi are able to withstand very warm and almost-freezing temperatures. When kept in backyard ponds, there are a few points you will need to consider to keep your koi happy and healthy when colder weather comes.

How to Keep Koi Fish Safe in Winter

If your fish reside in an area that can reach freezing temperatures, be sure to address the following checklist to ensure your fish will survive the winter.


The biggest consideration when over-wintering koi outside is to make sure their water will not freeze into one solid block of ice. Surface freezing is fine for most ponds. How many inches down your pond freezes will depend on your weather, so ensure your fish have enough room to safely tuck themselves under the ice. If your pond is overstocked, there is a risk that the fish will be packed in too tightly and run out of oxygen or produce too much ammonia, causing stress that could cause secondary disease.

Most koi ponds built into the ground are well-insulated by the soil; this allows for mostly a surface freeze. Above-ground ponds or temporary holding ponds are at risk of deep freezing. In order to allow for room for your fish, wrap insulation, dirt or blankets around your exposed walls. This will prevent your pond freezing at deeper levels.

Oxygen Access

Even in frozen ponds, your fish will need oxygen to survive. A solid-ice surface cuts fish off from life saving oxygen and can easily wipe out an entire pond. You must provide some access to the air at the water's surface in order to keep your fish alive. You can do this by installing a pond deicer, aerator or running water feature.

Pond deicers work by thawing a small area of your pond surface in order to allow for air to penetrate into the water. These need to be checked frequently in order to ensure proper function. They do not heat the pond, only a small area of the water at the pond surface,

Aerators work by keeping ice from completely covering the area directly above them. In very cold temperatures, however, the surface may still freeze over. Also, you do not want to bring the warmer water from the bottom of the pond to the surface using a bubbler on the bottom. These air stones should be placed just below the surface in the winter.

Running water features work similarly to an aerator by constantly disrupting the formation of a solid ice surface. These work slightly better than an aerator, but can cause supercooling if they are too large and can also freeze in very cold temperatures.

Supercooling Features

Air exposed to super cold temperatures, such as that within a tall waterfall, can supercool ponds by quickly dropping the water temperature, commonly as the sun goes down. A sudden decrease in temperature can negatively impact your koi and potentially kill them. If you have a tall water feature, it is recommended to bypass it during very cold weather conditions.


In colder water temperatures, your koi fish's metabolism will reach extremely low levels. If the temperature is going to drop below 45 degrees Fahrenheit/7 Celsius, it is not recommended to feed your fish. Your fish may beg for food when the water is warm, but it is best to keep their gastrointestinal tracts free of food that may stop moving through if the water is expected to get colder. Usually, koi are pretty good about not wanting food when temperatures drop, but if you still get hungry faces, tell them it's for their own good.

Can I Keep My Koi Fish Inside (link to other article)

Provided you have addressed the points above, your fish will be just fine outside all winter long. If you cannot provide the correct outdoor environment for your fish over the winter, you will need to bring them to a temporary indoor tub, commonly referred to as "over-wintering."

There are two critical components to successfully over-wintering your koi indoors.

Requirement 1: Enough Space

Yes, usually most indoor setups are severely space restricted. For most ponds, it is recommended to have at least 250 gallons per koi, which is considerably more than most koi owners think they'll need. Indoor holding tubs may be considerably smaller, since they are only temporary. For koi, the bigger the tub you can set up, the better. More water will decrease the amount of stress your koi are under and reduce the risk of secondary infections.

Requirement 2: Bring Your Filtration with You

The worst thing that can happen to a new indoor setup is New Pond Syndrome. This occurs with all new systems that do not have established biological filtration. All those "instant start" products on the pet store shelves are not effective. Very large fish in a smaller pond are at very high risk for deadly ammonia spikes.

The best thing you can do to ensure your fish will survive this transition is to bring their filter media along. This is usually either matting, strapping or plastic beads. By bringing these along to your indoor setup, and placing them into a filter for the indoor holding tank, the ammonia level in the water will not spike and your fish will be much happier.

How to Keep My Koi Fish Healthy During Cold Snaps

Temporary periods of cold water are possible for any pond. Even if it is rare in your area to get freezing temperatures, all koi pond owners should plan ahead. Be sure to address the key points above and be sure your pond does not freeze into a solid block by providing adequate insulation and that your fish have access to surface oxygen. Bypass all tall water features and hold off feeding. These are the best steps to take to ensure your fish will be okay.

Once the water warms up again, your fish may be a bit sluggish to get back to normal behavior and appetite. This is normal, and how long it takes to get back to their regular behavior depends entirely on your individual pond's filtration and water temperature. Sometimes, one or two fish will not get back to normal as fast as the others in the pond. Once you notice a is fish struggling, it is best to call your aquatic veterinarian immediately to ensure nothing gets worse.