Can Koi Fish Survive Winters in An Outdoor Pond?

Amazingly resilient creatures, koi fish have been known to withstand a wide range of temperatures. Koi can survive very hot summers and near-freezing waters in their native land, Japan.

It is this resistance to weather changes that has allowed koi to thrive all over the world, making koi-raising and breeding a popular hobby everywhere.

When keeping koi in outdoor ponds, there are a few things you have to keep in mind, to ensure that your pet koi fish are kept healthy and happy as the seasons change.

What Does It Mean to Winter Your Koi?

Koi fish are a breed of carp native to Japan. They originally inhabited areas that were known to reach freezing temperatures, but still managed to thrive. This is because, in cold temperatures, koi become dormant.

This is known as wintering. In man-made ponds, however, domesticated koi may not winter as naturally as those found in the wild, so koi owners and breeders need to take certain measures to help their koi survive the winter.

How To Keep Your Koi Fish Safe During Winter

A large koi pond full of beautiful fish can uplift any outdoor area. That said, if you happen to live in a city that gets nail-bitingly cold during the winter months, there are certain precautions you need to take.

This will ensure that during the winter your koi aren’t just safe, but thriving.

Provide Enough Space

Making sure that their water won’t freeze into a single, solid block of ice is the main factor to take into account while overwintering koi outside. For most ponds, surface freezing is acceptable.

Weather will determine how deep your pond freezes, so make sure your fish have enough space to securely hide under the ice.

If your pond is overpopulated, there is a chance that the fish will be crammed too closely together, run out of oxygen, or create too much ammonia, leading to stress that might result in sickness and death.

Keep The Pond Insulated

Since the soil and dirt in most koi ponds are thick and insulated effectively enough, it’s probably fine if the surface of the water freezes over with a thin layer of ice.

However, be careful of deep freezing that might occur in temporary holding ponds or above-ground ponds

Wrap insulation, mud, or blankets over your exposed walls. This will stop the deeper areas of your pond from freezing at depths that can endanger the well-being of your koi fish.

Keep The Pond Oxygenated

Your fish require oxygen to thrive, even in freezing ponds. If the surface of your pond freezes, it can deplete oxygen levels in the water, making it difficult for your pet koi fish to survive.

In this situation, it isn’t necessarily the freezing temperatures that kill koi, but the lack of oxygen.

To keep all your pet koi alive, you must provide them some access to the air just above the water. Installing a pond deicer, aerator, or flowing water feature will help circulate oxygen throughout the entire pond.

Apparatus That Keep Your Pond Oxygenated

Aerator: Aerators function by preventing the space directly above them from becoming totally covered in ice. But in extremely low temperatures, the aerator may fail to keep the surface of the water from freezing over.

Pond De-icer: Pond de-icers function by melting a small portion of the ice on your pond’s surface so that air may enter the water.

To make sure they are operating properly, these should be checked periodically. Only a tiny portion of the water at the pond’s surface is heated; the entire pond is not.

Flowing Water Feature: By continuously interfering with the creation of a solid ice surface, flowing water features function similarly to an aerator. However, it’s important to make sure your flowing water feature is the right size for your pond. If they are too big, they can cause the water to freeze almost instantly.

Don’t Use a Bubbler: novice koi keepers often make the mistake of installing a bubbler at the bottom of their koi pond. The purpose of the bubbler is to move warm water upward, from the bottom of the pond to the surface.

In theory, this would prevent the surface from freezing over. However, the side effect of this is that the temperature in the middle of the water drops suddenly, and this can harm your koi.

Don’t Overfeed During the Winter

Your koi fish’s metabolism will drop to incredibly low levels during the winter months. Most expert koi keepers advise not to feed your fish if the temperature drops below 45 degrees Fahrenheit/7 degrees Celsius.

Even during the warmer moments of an otherwise cold day, when your fish may clamor for food, it is better to keep their gastrointestinal systems clear—because food may cease passing through if the water temperature drops.

It might seem cruel to intentionally deprive your pet koi of food in the winter, but it is for their own benefit.

It’s not just the quantity of your koi’s food that matters, but quality as well. During the winter, their metabolism slows down, meaning they digest their food much slower. High-quality koi fish food will be easier to digest.

That is why experts recommend buying your koi fish the best food available, instead of trying to save money on cheaper food.

Why Koi Fish Die in The Winter

There are many reasons that koi fish may die during the winter. It’s important that you’re aware of these reasons, so you can take the necessary measures to prevent problems from occurring in the first place.


The number one reason pet koi fish die during the winter is neglectful owners. Raising koi isn’t just an expensive hobby, it’s also time-consuming.

You need to stay vigilant, and take time out of your day to check up on your koi, to make sure they are happy and thriving, especially during the winter.

Faulty Equipment

There is a lot of different equipment that goes into taking care of your pet koi fish, especially during the winter. These can range from air filters to aerators, deicers, and many more.

However, certain tools and gadgets tend to break down during the winter, as their inner mechanics are unable to survive extremely low temperatures.

During the winter months, regularly check that all the equipment that goes into keeping your koi pond running is functioning properly. You may not realize it, but even one item not working properly can endanger the lives of your pet koi fish.

Power Outages

Given the previous point of how important certain electronic equipment is in order to maintain your koi pond during the winter, it makes sense that a constant electric supply is absolutely necessary.

During the winter, however, power outages may occur. Power outages are more common in certain regions than in others.

Regardless, you should explore your options for alternate power supplies, to ensure that your pet koi fish continue to get their needs met, even when the power goes out.

Should You Keep Your Fish Indoors?

Some koi keepers prefer to move their pet koi fish indoors during the colder winter months. Is this entirely necessary? And what do you have to do to ensure your koi fish are just as happy indoors, as they were outside? Let’s discuss this.

If you have a garage, you could consider setting up a secondary tank there. Garages tend to be well-insulated, and stay warm during the winter. That means your koi will be safe, at the very least. What you’ll need here are a filter and a bubbler.

The size of the tank you’ll need depends on how many koi you own, and how big they are. Most koi raising experts recommend tanks with a capacity ranging from 200-1000 gallons.

If you don’t have a garage and intend to keep your pet koi in your home, you will need a larger filter and bubbler.

Bear in mind that by choosing to keep your koi fish indoors during the winter, you will be preventing them from hibernating. This means they will continue to grow during the winter.

Now That You Know

Now that you know so much more about what it takes to properly raise your pet koi fish during the cold winter season, what do you think?

Are you an experienced koi keeper? If so, do you prefer to allow your pet koi fish to hibernate outdoors during the winter?

What do you do to help them during this time? Or, do you prefer to place them in a secondary tank, in a warmer area, where they will be safe until the temperature gets a little warmer?

If you’re new to raising your own koi fish, hopefully, this article proved useful to you. Koi raising can be incredibly rewarding, but it’s also a massive undertaking.

It’s important to learn how to raise your koi properly, so you can give them the quality of life they deserve.

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