If you have an outdoor koi pond, you may be wondering how to keep your aquatic friends safe when the winter weather arrives. Koi are a cold water species, meaning their body temperature is primarily affected by the water temperature surrounding them. This is why providing your fish with an environment conducive to surviving the colder seasons is essential. 

Depending on your climate, you’ll need to overwinter your koi pond to keep your fish safe throughout the season. This may sound stressful, but it’s a feasible process – you can prepare your pond for chilly weather in just a few easy steps. We’ll cover these steps and more in this guide to keeping your koi fish safe in winter.

Tips for Overwintering Your Koi Pond

infographic explaining how to keep koi fish safe in winter
Image Source: Elisabeth Beauchamp / Today’s Homeowner

Assess Your Area’s Winter Weather

Knowing how cold your koi pond can get during winter is the first step to keeping your fish safe. The USDA Plant Hardiness Zone Map is a reliable tool for understanding your area’s cold weather trends. This map categorizes the U.S. into zones based on annual minimum winter temperatures. 

Alabama Aquarium & Pond Services recommends winterizing your fish pond if your area is zone 10 or lower. In these areas, temperatures drop below 35°F to 40°F, which is dangerously low for shallow koi ponds. If your pond is shallow or above ground, you’ll likely need to move the fish indoors for the season.

Moving Your Koi Indoors

For the most part, koi can survive winters outside if their ponds have been properly prepared. However, most shallow and above-ground ponds can’t maintain enough warmth for your fish to survive. Consider moving your fish indoors for the season, which is the best choice for their health and safety. 

Where To Move Them

Ideally, it’s best to move the fish to an unheated garage or shed that stays below 50°F but above freezing. This allows the fish to hibernate without the threat of becoming icicles as they could in their outdoor pond.

Prep the Pond

Before moving your koi fish from the outdoor pond, remove any debris or buildup with a pool skimmer. This quick cleanup helps to simplify catching your fish and removing the water needed for the temporary tank. 

Prep the Tank

Next, prepare your indoor tank for the fish. You’ll need a sizable tank that allows your fish to move freely without overcrowding. Pre-formed pond liners and aquarium tanks are two good options. No matter what container you choose, cover it entirely with a breathable net or frame. Koi are talented jumpers that need a barrier to prevent surprise escapes.

Before filling up the tank with a garden hose, remember that your fish are used to a natural aquatic environment. You’ll need to keep their temporary water similar to the outside pond to prevent them from undergoing stress and shock. Smartpond suggests filling the temporary tank with between 20% and 50% pond water to ease the transition. You can then add aquatic plants to the temporary tank to introduce beneficial bacteria into the water. The plants also help filter out fish waste toxins as they would in an outdoor setting.

Move Your Fish

Move your fish to the new tank slowly, allowing them to acclimate to the new conditions. One way to do this is to remove them from the outdoor pond and put them into a bucket of pond water. Once each koi fish is in a bucket, add water from the indoor container every 15 minutes until the bucket’s water temperature is the same as the indoor tank. Then, you can safely release the fish into the indoor container.

How To Move Koi Back Outside in Spring

Once the outdoor pond’s water temperature is consistently above 50°F, you can begin transferring your koi back outside. Since they’ve spent the winter hibernating in the temporary tank, you need to provide them with a smooth transition back to the outdoor environment. 

First, scoop them out of the indoor tank and into a temporary outdoor container filled with water from both the indoor and outdoor habitats. Then, slowly add pond water to the container until its water temperature is the same as the indoor tank. After your fish have had a while to adjust to the outdoor water, you can safely release them back into the pond.

Perform Autumn Pond Maintenance

There are a few housekeeping steps to take if you plan to overwinter your koi fish outdoors. Simple pond maintenance sets your fish up for success when freezing temperatures arrive.

Aquascape, Inc. offers the following fall pond maintenance tips to keep your fish safe:

  • Skim – Use a skimmer to remove leaves and debris from the bottom and surface of the pond. A simple cleanup will keep the water quality healthier while aquatic life enters dormancy. 
  • Trim – Clip nearby foliage and add pond netting over the water to keep debris from falling in. 
  • Medicate Add beneficial bacteria to help maintain the water quality throughout the cold season. Cold water bacteria work through the winter to digest the grime and debris that can plague your pond.

Remember, what you do in autumn can really benefit both your pond and your fish through the winter and into the spring.

person cleaning debris out of a pond before winter
Image Source: Canva

Adjust Feeding Schedule Based on Temperature

Like many other fish species, koi enter hibernation during the winter when water temperatures fall below 40°F. Their metabolism and immune systems slow down, allowing them to conserve energy in unfavorable living conditions. 

You might think your fish need a little extra nutrition during this time, but that’s not true. According to Ultra Balance Koi Food, koi fish are cold-blooded, so their metabolism is especially susceptible to temperature changes. The temperature drops when the colder weather arrives, and the fish progressively slow down until they reach complete hibernation. In other words, the cold weather halts the koi’s digestive system. 

Feeding Temperature Guidelines

Use the following guidelines to adjust your koi feeding schedule throughout the year:

  • Above 64°F Feed your fish a few times a day to help them maintain growth. They’ll be most active during warm weather, creating a higher need for energy.
  • Below 64°F – Give koi an easily-digestible fish food like wheat germ. This food is gentler on their digestive systems as they acclimate to cooler weather. 
  • Below 50°F Stop feeding altogether at this point. This is when your koi should start spending most of their time hibernating near the bottom of the pond. They’ll bottom-feed on algae to get the necessary nutrients.
hand feeding a koi fish some food
Image Source: Canva

Ensure Adequate Oxygen Flow and Filtration

Many pond owners whose koi fish die during winter think freezing temperatures must be the culprit. However, this isn’t always the case. Sometimes, imbalanced oxygen levels are more detrimental to your scaly friends than cold temperatures.

Ponds are full of organic matter from plant and fish waste. During most of the year, this matter breaks down naturally, producing gases that evaporate from the water’s surface. However, if the pond freezes over in the winter, the buildup of toxic gases has nowhere to go. At the same time, oxygen won’t be able to enter the pool if it’s completely covered with ice. 

As frigid winter temperatures settle in, ensure the entire pond doesn’t freeze over. One way to do this is to install a pond heater or de-icer to keep the pond’s surface from freezing shut. The device won’t circulate warm water throughout the entire pond but instead warms the surface enough to keep an opening in the ice. 

This opening is crucial to your fish’s safety, allowing gas exchange between the pond water and outside air. Meanwhile, your koi will head to the bottom of the pond, where natural insulation allows them to hibernate safely for the season.

In addition to keeping an opening in the ice, use an aerator to keep oxygen flowing throughout the pond. Aerators, sometimes called “bubblers,” move oxygen through the water while preventing standing surface water from freezing solid. Bubblers like this one from AquaMiracle include air stones, another must-have tool for your winter koi pond. 

Air stones are dense, sponge-like devices that connect to an above-water air pump. The pump pushes air through a tube into the stones to produce numerous bubbles. The flow of bubbles into the pond increases the water’s oxygen content, improving its overall filtration and aeration.

With these tips in mind, you may think that leaving fountains and waterfalls on during the season is an ideal way to keep water and air flowing. However, this isn’t necessarily the case. Many pond keepers suggest turning off water features because they can throw off the pond’s natural insulation. 

For example, suppose you keep a waterfall running throughout the season. Any external temperatures affecting that water supply are essentially dumped into your pond, disrupting the fish and plants hunkered down for winter.

Your best bet is to turn these systems off for the colder months. If your area reaches especially cold temperatures, you should also cover air pumps and pond filtration systems with a tarp to prevent them from freezing.

Looking Ahead to Spring

Now that you know how to prepare your koi pond and move your fish for the winter months, you’ll have happy and healthy fish come spring. As temperatures rise and the ice melts from your pond, get ready for your fish to be active once again. 

When the water temperature stays consistently above 50°F or 55°F, move your koi from their temporary indoor closure back into the outdoor pond and reintroduce easily digestible foods into their diet. You can adjust the feeding schedule as the water warms up to help your fish resume activity. Soon enough, your koi fish will be back at their full activity level and ready to enjoy their habitat during the warmer months.

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Editorial Contributors
Elisabeth Beauchamp

Elisabeth Beauchamp

Senior Staff Writer

Elisabeth Beauchamp is a content producer for Today’s Homeowner’s Lawn and Windows categories. She graduated from the University of North Carolina at Chapel Hill with degrees in Journalism and Linguistics. When Elisabeth isn’t writing about flowers, foliage, and fertilizer, she’s researching landscaping trends and current events in the agricultural space. Elisabeth aims to educate and equip readers with the tools they need to create a home they love.

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Lora Novak

Senior Editor

Lora Novak meticulously proofreads and edits all commercial content for Today’s Homeowner to guarantee that it contains the most up-to-date information. Lora brings over 12 years of writing, editing, and digital marketing expertise. She’s worked on thousands of articles related to heating, air conditioning, ventilation, roofing, plumbing, lawn/garden, pest control, insurance, and other general homeownership topics.

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