Winter Care for your Koi Pond

When winter begins to move in and temperatures drop, Koi move into the deeper parts of the pond, where warm water will be able to remain after the surface freezes, and they become more reluctant to feed. Once the temperature drops below 43 degrees Fahrenheit, they will stop eating entirely. As the water continues to cool, the Koi will tend to gravitate to one another and lie huddled together at the bottom of the pond with all their fins (minus the obvious tail fin) clamped tightly against their bodies. This conserves body heat, which is important during the winter season. Once temperatures really drop, down in the 36-43 degree Fahrenheit range, Koi go into what is described by some as minimal type state. Their senses such as smell
and taste are completely inactive but they can still see and respond to some external stimulation, such as touch, or water movement below the surface.

Winter filtration and aeration are extremely important tasks to undertake to make sure your Koi survive the winter in good shape. While usually heat rises and cold stays low, the opposite happens when water freezes. The surface freezes, and warmer water settles to the bottom, which is what allows
the Koi, and other underwater fish, to survive. This is part of the reason you want your pond built a minimum of three feet deep: because this allows (in colder climates) for the top layer to freeze, insulating the warmer water on the bottom so the Koi have enough warmth, and room, to survive.

Keep all your biological filters running in the winter. It is important for the water to continue to ventilate, so that oxygen can diffuse into the water and harmful carbon dioxide does not become sealed in by ice. If the pump is turned off, the fish will suffer and the plumbing will corrode, or freeze. If you have a variable flow pump fitted in the pond, turn it to its minimum output. If you have a submersed pump in the pond, you can vary the level from which the filter is fed. As the water temperature drops towards the critical 39 degrees Fahrenheit, raise the pump or filter feed to 6-9in below the surface. This will avoid disturbance and chilling of bottom water and allow the Koi to rest, while preventing the pond from completely icing over.

It is not a good idea to use rubber balls, hammers or kettles of boiling water to prevent or break up ice formations. Although the Koi are torpid, they can sense any water movement, and breaking ice sends violent shock waves through the water, which can severely stress, and even kill, Koi.

A recent popular trend has been to outfit the outdoor Koi ponds with heaters that can be used to help regulate the temperature during the winter. Depending on how much you choose to keep the pond heated, this also gives you the ability to keep your Koi active and feeding, and therefore growing, throughout the year.

If you choose to go this route:

Do Not place heaters in the bottom of the pond. This can result in convectional currents that lift the warmer water, which chills as it rises towards the surface, thus returning cold water to the Koi. You should take away any air stones from the bottom of the pond in winter for the same reason. Place small heaters in the filter system, ideally in the settling chamber or transfer ports. Any heating unit should be thermostatically controlled and capable of maintaining the water temperature at a minimum of 50 degrees Fahrenheit, even through the severest of winters. Rapidly fluctuating temperatures can be extremely harmful to Koi, and are worse for them then normal winter treatment in a Koi pond without additional heaters. If your heater cannot maintain the needed temperature throughout the winter, use it only to help stabilize autumn and spring temperatures.