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How to Keep an Outdoor Fish Pond Warm for Winter

Nat Howard

A koi pond is a delicate ecosystem that needs maintaining even in the winter months. Pond depth and size play a part in the heating device needed to keep koi fish warm for winter.

How to Keep an Outdoor Fish Pond Warm for Winter

When winter hits, it’s tough to tell if your koi fish are fine or frosty. Take time this winter to prepare your koi pond for chilly weather and ensure your fish survive and thrive until spring.

Shallow Ponds

Outdoor pond depths can vary, and depth can affect the health of koi fish in colder temperatures. Deep ponds will not need any extra heating measures. Fish will survive through the winter if the pond is deep enough so that the water does not completely freeze. Some sources recommend 3-to-5-feet deep, and others recommend 6 feet. If your pond is shallow or less than 2-to-3-feet deep, you’ll need to take protective measures.

Gas or Propane Heater

A gas-fired boiler is a heater powered by gas or propane. It’s used for larger, wider fish ponds that hold thousands of gallons of water. If your pond is small, the gas-powered boiler may be an expensive installation and may not be necessary. The boiler pumps pond water through a stainless-steel heater and releases the warm water back into the pond. This type of heater needs professional installation.

Electric Heater

Electric heaters are best for medium-sized ponds and ponds used for breeding. If your pond holds less than 1,000 gallons of water, be careful when choosing an electric heater. Water can heat up too quickly, overheating and harming the fish. Electric heaters, like gas heaters, use a pump system. Electric heaters run along the existing pond pipes and installations are often beside the pond pump. They pull cold water into the heating chamber while letting warm water run out, gradually warming the pond.

There should be a minimum of one kilowatt (1,000 watts) for every 1,000 gallons of pond water. A larger 3,000-gallon pond will need an electric heater with 3kW to keep the water warm. Some outdoor pond heaters have efficiency settings that run only when the water temperature approaches freezing.

De-icers and Aeration

A de-icer is a heating device that floats on the surface of the water and allows the pond to “breathe” when naturally produced gases under the ice release into the air. This also gives oxygen to your fish which are hibernating at the bottom of your pond. Ice on the pond can provide a protective cover but air needs to get in to keep a steady temperature and stable ecosystem. Be aware of your gallon size when buying a de-icer. A pool of around 250-to-500 gallons is the right size for a de-icer. If your pond is small, the heat may harm the fish.

An aeration system works in much of the same way by moving the water and creating holes in the ice so that the pond does not completely freeze. You can buy and install this system yourself. Replace air filters every three-to-six months, depending on the system. Aeration systems should be closer to the top of the pond in winter months. For a small pond, this is your best bet.