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Checking in on Your Pond Fish During the Winter

Checking in on Your Pond Fish During the Winter

When winter comes around, if you have fish in your outdoor pond, there are two options: move your fish inside or let them overwinter in the pond. Whether or not you can overwinter your fish outside depends on how deep your pond is and how cold your winters are. If you have a shallow pond, you can relocate your fish indoors in a temporary tank until warmer weather in the spring. During the winter, it's important to keep a close eye on your pond fish to make sure they're healthy and catch any potential issues before they cause problems. 

Overwintering Outside

Fish can overwinter in ponds that are deep enough not to freeze all of the way through. The water at the deepest part of the pond actually becomes the warmest. As the weather cools, your fish need less food (eventually no food at all), and they become less active. 

What to Keep an Eye out for When Checking on Your Pond Fish

Is a Section of the Pond Surface Unfrozen?

When the pond's surface completely freezes over, the water is unable to oxygenate—the gasses created during the breakdown of waste become trapped, causing stress and harm to your fish. Pond fish are less active during winter but still need fresh oxygen. Keeping a section of the ice unfrozen lets the water oxygenate.

An aerator is one of the best ways to keep a section of the ice open. Move the aerator to the side of the pond, in a more shallow area, to prevent overcooling. In addition to oxygenating the water, it also keeps the surface water agitated so the ice can't freeze as quickly. If the ice keeps freezing, you can use a de-icer to thaw the area.

The section of ice could freeze over for a few different reasons. A cold snap could cause the temperatures to drop drastically. Snowstorms can lead to power outages, preventing the aerator and de-icer from running without a generator. Your aerator or de-icer could also break or need maintenance. Unless your pond is right outside your window or along a pathway you walk every day, it's easy to forget about. The section of the ice could freeze over, and you'd only know if you stopped by to check on it.   

If the section has frozen over, the best way to quickly unfreeze it is to carefully pour hot water over the part you want to reopen. Never use a pickax or force to reopen a section of ice, as the loud breaking sound is amplified in the water and can cause stress and harm to the pond fish.

Conduct Water Tests

While everything may seem peaceful above the surface of your pond, signs of an imbalance in the water during the wintertime are often not always apparent until it starts harming the fish. Water quality tests will tell you if there is too much ammonia or nitrites/nitrates and if the pH levels are within the proper range. In a shut-down winter pond, there is less water than normal for the fish to swim around in (which is fine because they are less active), but that means that it's easier for water quality issues to occur.

Remove Snow

Snow that buildups on the surface of the pond can prevent sunlight from penetrating the water. At least 25 percent of the pond's surface should be snow-free to help keep the water oxygenated. The tiny plants in the pond need sunlight to photosynthesize and oxygenate the water, which is essential for water quality and fish health.

Remove Debris

The good bacteria in the pond are less active during winter than in other seasons. The pond's nitrogen cycle is less efficient during this time, which is why it's essential to keep your pond free of debris in the fall. Leaves, branches, and other organic materials can find their way into the water. They are not broken down quickly and sink to the bottom of the pond (where your fish are), decay, and slowly break down. The water quality and oxygen levels can suffer as a result.

Sick Koi

As the weather gets colder, a fish's body begins to slow down. Their metabolisms come to a near halt as they live off fat reserves until spring. Their immune systems are less active, making it harder to fight off disease, parasites, infections, and wounds. 

Many parasites and bacteria can survive the cold temperatures and prey on your fish when they're the most vulnerable. If you see just one or two sick fish, you can treat them separately, but the whole pond needs treating if multiple fish are sick.

Many fish medicines are not as effective in cold weather once the water drops below 50°F. There are certain things you can do, such as adding salt to the water or special cold-water anti-parasite medications for your fish. Alternatively, you can move them inside and treat them in warmer water. 

Warming Up

During the wintertime, there can be fluctuations in temperatures. After a couple of weeks of cold weather, it may warm up for a few days or even a week or two. The temperature changes can be stressful and dangerous to your pond. 

When the water warms up, fish become more active, and it can lead to a spike in ammonia in the pond. During a warm spell, it's essential to perform a water test and treat any water quality issues quickly. Another concern during a warm period is an increase in parasite activity. 

Certain parasites like flukes will lay eggs and stay dormant during the winter, waiting for the warmer weather to become active again. A warm spell tricks the dormant parasites into thinking it's time to wake up, and they can harm your fish. Be on the lookout for any signs of parasites during a warm spell, and treat them as soon as the symptoms start showing.


A pond owner's work is ongoing; there's always more to do! During the winter, there is typically less gardening and pond maintenance required compared to the other months of the year, but it's essential to keep an eye on your pond. When you're indoors enjoying a warm fire and hot cocoa, it's easy to forget about the fish in your shut-down pond. During the winter, you should check on your fish periodically to make sure everything is running smoothly, perform water tests regularly (and during a warm spell), and see if your fish are acting normally. It's best if you can catch any issues before they cause major problems. Soon enough, spring will arrive, and you will be able to spend more time with your active fish.



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