Prep Your Koi Pond for Winter so Your Fish Can Thrive

You may enjoy the chill in the air as the weather turns colder, but will your koi? Here's what you need to know to keep your fish safe all winter.

Published August 15, 2023
Koi carp in a pond

Koi add a zen and colorful touch to your yard or garden during the warmer months, but what about during chilly winters? If you have a koi pond in an area where the winters are freezing, you may be concerned about their survival. Fortunately, koi are hardy fish that have adapted to withstand cold temperatures. Despite their hardiness, however, there are times you may want to consider a temporary indoor pond or aquarium.

How Koi Survive Winter

As the days grow shorter and temperatures plummet, koi instinctively prepare for the coming chill. Their metabolism drastically slows down. This biological shift means that your koi will require significantly less food. In fact, it's advisable for pond owners to reduce feeding as temperatures drop and to stop entirely when the water consistently hovers below 50°F.

Koi fish don't swim near the surface very often during the winter. Instead, they tend to stay in the deeper parts of the pond, where water temperatures remain relatively stable and warmer. They will spend most of the winter in a semi-dormant state to conserve energy.

Steps to Winterize a Pond for Koi

As winter approaches, it's important to take the steps necessary to keep your koi healthy.

Clean the Pond

Autumn leaves and organic debris can accumulate in the pond. If left untreated, they decompose, reducing water quality and oxygen levels. This isn't ideal for wintering koi. Use a pond net or skimmer to remove leaves and debris. Consider adding a protective net over your pond in the fall to prevent excessive leaf buildup.

Check and Adjust Water Levels

Ensure your pond is as full as possible going into winter. This maximizes the depth for the koi, providing them with a larger area of warmer water at the bottom. The pond should be at least 18 inches deep, but three feet or deeper is ideal, especially during freezing temperatures.

Need to Know

The deeper the pond, the more likely that your koi fish will be content, and the higher their odds of survival through the winter.

Add a Heater

Adding a heater is recommended, especially if your pond is on the shallow end of the spectrum. Place the heater as low as possible, or on the bottom of the pond if it's shallow enough, and set it to 50 degrees. At 50 degrees, your koi will probably continue to eat, but they won't eat as much as they do during the warmer months.

Need to Know

Above-ground ponds will freeze faster than natural ponds that have insulation from the soil.

Maintain Oxygen Levels

Install an aerator or pond bubbler. This equipment introduces essential oxygen into the water and promotes a healthy gas exchange, ensuring harmful gases don't accumulate under the ice.

Keep a Hole Open in the Ice

Image of large frozen koi pond in winter garden covered with ice and snow after heavy snowfall

It's crucial to ensure that the pond doesn't freeze over completely. You can achieve this using a floating pond heater or a de-icer. The de-icer doesn't have to heat the pond, it just needs to create a hole in the ice to allow air to penetrate the water.

Quick Tip

Check your de-icer regularly to make sure it's functioning properly.

Turn Off and Store External Pumps and Filters

If you use external pumps and filters, it's usually a good idea to turn them off, clean them, and store them for the winter to prevent any damage. Some pond owners keep their pumps running to aid in water circulation, but make sure it's deep enough, so the pump doesn't freeze.

Consider Using Beneficial Bacteria

Adding cold-water beneficial bacteria can help break down organic materials and improve water clarity and quality, setting the stage for a healthier environment for the koi.

Keep a Watchful Eye

Throughout winter, periodically check on the pond. Ensure the hole in the ice remains open and keep an eye on the overall condition of the pond and the health of the koi. A floating heater de-icer can keep the pond from freezing, which may prevent you from having to bring the koi inside.

When to Bring Koi Inside

Koi pond in winter with snow

While koi are notably resilient and can endure colder temperatures in outdoor ponds, there are instances where an indoor retreat becomes necessary. If you're living in an area where winters are exceptionally harsh and pond temperatures might dip perilously close to freezing, or if the pond is not deep enough to offer a stable, warmer zone for them at the bottom, it's a wise move to consider an indoor setting. Additionally, if there's a forecast of extended periods of icy conditions that could risk the pond completely freezing over, bringing your koi inside to a controlled environment, like an indoor pond or tank, ensures they remain safe and healthy.

Need to Know

Relocating koi is stressful for them, so it's essential to make the transition as smooth as possible.

Setting Up a Temporary Indoor Aquarium

If you aren't comfortable leaving your koi outside in the pond, or your pond isn't deep enough, consider setting up a temporary indoor aquarium until the temperature goes back up. To set up the aquarium properly, consider these factors.

  • Tank size: Choose a sizable tank; koi need space to swim and grow. While they'll be spending less time zipping around compared to their outdoor antics, they still appreciate room to stretch their fins.
  • Filter: Equipping the tank with a powerful filter is a must; koi, as much as we love them, are known for producing a significant amount of waste. You'll want a filtration system that can keep up!
  • Heater: A water heater might be necessary, especially if the room's ambient temperature isn't ideal. Aim for a stable temperature range of 65°F to 75°F.
  • Air pump: A good air pump and air stone will ensure the water remains oxygen rich.
  • Decor: While decor isn't incredibly important for short-term stays, a few sturdy plants or hides can offer some comfort and semblance of their outdoor home.

With these elements in place, your koi will have a cozy indoor retreat to tide them over until they can get back outside.

Plan Ahead

Even if you think your pond is deep enough to keep your koi fish healthy through the winter, you should still have a plan in place. If you have a particularly harsh winter with a long cold spell, you may still need to bring your fish indoors temporarily. It's better to be prepared than to be frantically collecting materials if the time comes.

Prep Your Koi Pond for Winter so Your Fish Can Thrive