Will My Koi Pond Freeze in Winter? (Frost Protection Guide)

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Will My Koi Pond Freeze in Winter? A Guide to Frozen Pond Protection 

fish pond frozen in winter with duck
Ponds freezing in winter is natural, but steps can still be taken to ensure fish are more comfortable and safe through the season. Public domain.

Winter is coming…to garden ponds! It’s a season that affects all pond owners in the northern hemisphere, and is something we need to properly prepare for as koi or goldfish keepers. You certainly can’t avoid winter, and some countries will also get it worse than others, but there are still things all of us can do to help our fishy friends and ponds through the coming freeze.

As a matter of fact, winter can actually be quite beneficial to fish ponds, so long as you correctly prepare and “winterize” the pond beforehand. For example, the biggest threats to fish over winter are a lack of oxygen and harmful substance build-up, but these can be solved with correct preparation and equipment. If you are prepared, a frozen pond is actually a good thing as it helps insulate the water and create a more stable environment for fish to enter torpor (“hibernation”).

Obviously, a pond that is frozen solid is not a good thing (especially for fish), but a frozen pond surface is nothing to worry about if you are ready for it. In this article, we cover common questions new pond owners may ask during their first winter, as well as provide helpful tips for safely removing ice, creating air holes, and providing oxygen to pond fish.

When will my pond freeze? How fast does a pond freeze?

is it alright for fish ponds to freeze
When and how fast your pond will freeze will depend on where you live and your winter climate. Public domain.

When and whether your pond will freeze will depend on your location, but a useful indicator is the USDA hardiness zone metric. Although primarily used for plants to determine how they will thrive in a certain location, it is also helpful for determining just how cold your winters will get as it provides an average minimum winter temperature.

In general, zones 13-10 are unlikely to get any significant ice on ponds, and can happily keep away small amounts by simply running pumps/features all season. Zones 10-1 are more likely to have problems with ponds freezing, with the lower zone locations having particularly harsh and cold winters. Finding your pond’s location on the hardiness zone map is nice to give you an indicator of what to expect, and we suggest winterizing a pond for all locations in zone 10 and below to be safe.

Knowing exactly when a pond will freeze is difficult as every year is different, but we personally recommend starting preparation at the end of autumn for most pond owners. This will usually give you a few weeks until the first frost arrives, which should be more than enough to get things ready for colder weather. In warmer zone locations, ponds will freeze more gradually as winter arrives, but in colder zone locations, ponds can freeze literally overnight if a cold front comes in rapidly. If you’re in a colder location, preparing for winter at the end of autumn is even more important to ensure you don’t get caught in early ‘freak’ weather unprepared.

Why does a pond freeze from top to bottom?

Garden ponds, as well as all-natural bodies of water, will always freeze from top to bottom because the water exposed to the air loses its heat much faster in comparison to the water at the bottom. As the surface water loses its heat it begins to freeze, which creates a layer of ice on the top of the pond. The reason most ponds won’t completely freeze solid is because the top layer of ice now helps protect the deeper layers of water from further contact with the colder air. There will still be heat loss taking place, but usually, the ice is enough to create a barrier to protect the rest of the pond from freezing solid.

Since water is a much better insulator in comparison to air, the pond water will be much warmer under the ice compared to the air above. If you have pond fish, you’ll likely see them swimming around the deepest point of the pond where the temperature is the warmest and the pond is the most insulated. So long as the pond is deep enough, and you’ve the right equipment in place (more on this below), even in the coldest locations it’s unlikely a pond will ever freeze completely – which is a good thing for your fish!

Can Goldfish and Koi Survive A Frozen Pond?

koi and goldfish survive cold water in winter
Koi and goldfish can survive in very cold water, but they cannot survive if the pond water completely freezes solid! Public domain.

Both koi and goldfish can survive in very cold water, but they cannot survive being frozen solid in ice. If you think your pond will freeze solid, which may happen with smaller ponds, we advise fish be brought inside or moved somewhere warmer, such as a greenhouse. It’s very unlikely that larger ponds will freeze solid, especially with the correct winter equipment in place (heaters/aerators), but it could happen for small or pre-formed ponds in colder locations.

Since most smaller ponds will likely have goldfish or fry (baby fish), it’s easier for them to be brought indoors to prevent them from freezing in the pond. If you have koi carp, they’re likely in a much larger pond so there is much less chance of the water freezing solid. In this case, simply adding a quality heater and aerator to the water and making sure the pond is properly winterized is all that is needed for them to survive comfortably.

When water temperatures reach 50ºF (10°C), koi will stop eating and begin a process called “torpor,” or more casually, “hibernation” (though torpor is not actually the same as hibernation), which allows them to survive on very low energy requirements. So long as the water under the ice doesn’t freeze solid, and there is an air hole for them to breathe, they should be able to get through the winter without issue as they sleep off the season.

Should I break my pond ice in winter?

pond heaters create openings in ice for gas exchange
Pond heaters are a much safer way to create holes in pond ice, and will also stop ice re-forming afterwards.

If your pond is already frozen and you have fish, you should create an opening in the ice for gas exchange to take place. This will allow oxygen to dissolve into the water and harmful gases, such as carbon dioxide, to escape into the atmosphere. However, you should not simply break the ice with a hammer or tool, as this will cause all sorts of stress on your pond fish. The sound and energy from an impact to the pond ice will carry very strongly through the water, so this is something that should be avoided!

The best way to create a hole in pond ice is to purchase a pond heater/de-icer unit and let it sit on the top of the pond. As it heats up it will gradually sink into the ice and eventually reach the water underneath. It will then float on the water and keep the hole open all winter for gas exchange to take place. This method is much safer than breaking the ice physically, and also ensures the ice does not quickly reform afterwards.

Will my pond pump freeze in winter? 

how to keep a pond pump from freezing in winter
Pond pumps should be turned off and covered to prevent damage from ice and snow during winter. Public domain.

If you live in warmer climates and have milder winters you may be able to run your pond pump and equipment all winter. This has several benefits, such as providing winter filtration, oxygenation, aeration, and also helping to prevent ice from forming on the surface water. If your winters are not very cold (never getting below 32°F/0°C), the heat the equipment produces should be enough to stop water in the system from freezing and damaging equipment. 

However, for most locations, temperatures are likely to drop far below the point of freezing. In this case, pond equipment such as pumps, water features, and filters should be turned off and covered for protection. Water that freezes inside a pump or filter will expand, and this can cause serious damage to the internal structure of the equipment. Pond pumps in particular are easily damaged by ice, so they should be turned off and covered as soon as temperatures start to drop towards freezing.

As long as the pond has been prepared for winter your fish should have no issues surviving without filtration and water flow, so having a pump running is not a necessity, anyway!

Protecting Koi Ponds During the Winter Freeze & Removing Ice

Note: These steps make up just a small part of “winterizing” a fish pond and apply more to ponds already in winter or with frozen surfaces. For our full guide on winter care, including the build-up to winter and what to do, check our dedicated article here!

Step 1: Install a Winter Pond Heater 

winter pond heater One of the best ways to protect your pond from freezing over completely is to install a simple de-icer/heater device. These are capable of melting a small hole in the ice and keeping it open all winter to allow gas exchange to take place. Heaters come in two forms: the first being a floating surface de-icer and the second being an in-line heater. Floating pond de-icers are cheap to purchase, work great for fish, and can also heat up the surrounding water to give the pond a little extra warmth. These are our recommended choices for most types of ponds, as an open hole is usually all you need for you fish to survive and be comfortable in winter.

In-line heaters are the more heavy-duty option which are connected to your pond circuit and are actually designed to increase the overall pond temperature by a few degrees.  These sound like a good choice, but fish can happily survive very low temperatures without problems so long as they have oxygen and a healthy environment. Raising the water temperature requires a lot of electricity, and just a few degrees of warmth isn’t going to make a major difference if everything else is in check. For koi breeders or those with show koi, in-line heaters may be an option as keeping temperature stable is important for maximum health, but for the everyday koi keeper, they’re not really a necessary investment.

For most ponds with fish, a simple floating de-icer will be all that’s needed to keep your koi and goldfish safe. We recommend installing it before water has a chance to freeze, although it can be added on top of ice and it will slowly melt the surroundings and sink into the pond.

Step 2: Install a Winter Aerator (“Air bubbler”)

winter pond aerator air bubblerAnother important step to take if you have koi or goldfish in your pond is to install a method of aeration, with the easiest being a dedicated air pump. This isn’t required for ponds in warmer locations which may not have much ice build-up, but it’s almost essential for ponds in colder climates where water freezes over easily. Since you’ll be turning off your regular aerators in winter, such as pumps and water features, oxygen content in the water will be reduced and can decrease sharply with heavy ice build-up. Even though creating an air hole using a pond heater is often enough for gas exchange to take place, if you have a heavy stocked koi or goldfish pond, they may need more oxygen to survive comfortably under the ice.

Unlike regular water pumps which can freeze in colder weather, air pumps don’t require any water to function so won’t freeze up during winter operation. This makes them perfect for providing plenty of oxygen to pond fish trapped under a frozen pond, and the movement they create in the water also helps prevent ice forming – which is a bonus!

The number 1 killer of pond fish over winter is lack of oxygen and a build-up of harmful gases which can’t escape due to ice formation, so having both a heater and an air pump ensures your fish are much safer even during the coldest months of the year.

Step 3: Turn Off & Cover Pond Equipment

protect pond equipment in winterAs mentioned briefly above, most regular pond equipment you’ve been running all year will now need to be drained and switched off to prevent damage from ice and snow. Pond pumps will need to be drained, as well as filter boxes and water feature attachments. This should be done as soon as temperatures start getting close to freezing, and depending on your location, this could be as soon as the end of Autumn, or as late as early December. So long as you have started the process of winterizing your pond beforehand, switching off equipment early should’t cause any issues for your pond or fish.

After things have been drained, pond pumps should be either covered or brought indoors for safety. In most cases, filter boxes can remain outside without issue, but pumps can easily become damaged internally from even the smallest amount of ice and snow. Placing them in your garage, shed, green house, or under a protective cover is always best practice.

Once everything is switched off, the only things you should have running all winter are your pond heater and any air pump/aerators you have installed. These are designed not to freeze up easily, and can happily function all season without requiring any extra maintenance!


6 thoughts on “Will My Koi Pond Freeze in Winter? (Frost Protection Guide)”

  1. What are your thoughts on using de-icing cables around the perimeter and slightly below water level to help keep a Koi pond from freezing? They are low cost and I am wondering as I have not heard anyone talking about this method in my Google searches.

    • Hi Tabb,

      Thanks for the great question!

      Honestly, I’ve never considered using this method myself, but it’s an certainly an interesting thought.

      If I were to try this, my main concern would probably be related to the cables heat transfer potential. With de-icing cables, depending on their power output, you may find they’re only capable of keeping the very edges of the pond free from ice. The issue here is that fish are more likely to congregate towards the center of the pond in winter anyway, as it’s both safer (from predators) and better at retaining heat than shallower water. In this case, if you were not looking to actually ‘heat’ the pond, a regular floating de-icer would be easier and more effective. Also, another possible issue is if they’re placed too close to the pond liner or surface edge, you may even lose significant heat to ground frost, reducing overall efficiency.

      However, if you have a smaller pond where there is significantly less distance from edge-to-edge, heating cables as a de-icing solution may be viable for keeping more surface ice away in comparison with a similarly rated floating de-icer.

      Again, this is not something I’ve tested myself, so I don’t know for sure. Those are my initial thoughts, however. Thanks for the interesting question!

    • Hi Cliff,

      Yes, if you have particularly cold winters and can’t keep things running, submerged pumps can simply be turned off and left under the water over winter. Just be sure they’re deep enough in the pond so they don’t freeze solid, as this will cause damage to the internals.

  2. Am I right in thinking that filters are not needed in winter as fish produce no waist in hibernation. Is it necessary to test water in winter?

  3. Hi Pond Informer,

    I’ve noticed my koi pond water levels drop significantly in the winter. We’ve gone as far as taking the fish out to try to find a hole in the liner and nothing. We’ve since turned the pump and waterfall off and it hasn’t dropped since. Any idea why this might be happening? We don’t have this problem in the other seasons.


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