Overwintering Koi Under Thick Ice

Overview of two ponds each with their own bucket containing the air pump. There is about 12
Overview of two ponds each with their own bucket containing the air pump. There is about 12″ of snow on the ice surface. Note that there is very little disturbance to the ponds checking the aeration points. In the foreground, you can easily see my footprints going out to check the ice dome that is mostly covered by snow.

The winter of 2009 was brutal in the Midwest. The lows on January 15th and 16th hovered around minus 21° F. in central Iowa. To overwinter koi in outside ornamental fish ponds in these conditions is something a lot of koi owners have never experienced. It can be done, however, and it is not that complicated.

Three Techniques

There are three basic techniques for overwintering koi where there are heavy icing conditions.

• The koi can be removed from their pond and brought inside a heated building or greenhouse.

• A structure can be built over their pond that collects heat from solar radiation, and/or heat can be added.

• The last technique and the easiest by far, is to leave them in their pond with the addition of aeration, and let it ice over naturally.


Most koi owners are very nervous about keeping koi outside under ice. This is a very “touchy” subject to serious koi hobbyists. They simply will not trust leaving their valuable fish in these conditions. They may not see the fish for three months, and when the winter winds are howling and it is minus 20° F., most hobbyists fear the worst. To make matters worse, there are articles warning that water colder than 39° F. can be lethal or at the very least, deform koi. Unfortunately, this kind of information is misleading and not factual, and, in fact, the ice layer actually protects them from the brutality of winter.

Koi (a variety of carp with fancy coloration and scalation) need a cold rest period in their seasonal cycle. They have evolved to endure cold water winter conditions. This cycle includes a gradual acclimation to the onset of winter and cold water over time. This is naturally done in the late summer and fall when, as the weather cools, they become acclimatized to very cold conditions. Taking a koi that has been in 60 to 70° F water and immediately placing that fish under ice in a winter pond would most likely prove lethal.

We can get some clues as to how our koi can overwinter by observing carp that overwinter just fine in Midwest lakes and streams. Streams will have open water in the channel where the water flows fast. It is churning and mixing the entire water column all the time. The water is continually being exposed to subzero air temperatures. The actual water temperatures in these streams are very close to 32° F. In my own aerated 10,000-gal koi pond system that is 5.5´ deep the bottom temperature was just above 32° this past winter. I have checked my thermometers for accuracy and they are correct. All of my 38 large koi survived the winter in great shape. I have been using the same technique for the last 17 years and have always been successful.

How to be Successful

So, just how can a serious koi hobbyist successfully overwinter their fish under thick ice? Let me explain some steps that will work for you.

This is an aeration point that is domed over that is more easily seen since the deep snow is not present. Notice the large air bubble under the ice and air escaping the ice in the foreground.
This is an aeration point that is domed over that is more easily seen since the deep snow is not present. Notice the large air bubble under the ice and air escaping the ice in the foreground.

Pond design is always important for many reasons but having the right design for overwintering fish is rarely discussed. The best design for a koi pond is one that is open without a lot of convolutions to the edge. Avoid overcomplicated designs, such as islands, channels, peninsulas and similar additions that can create what are called “dead zones.” These zones are exactly that – DEAD – devoid of life due to low oxygen (O2)! It is difficult to adequately circulate the water in these areas and consequently they usually have lower O2 levels. They are likely to be lower in O2 all times of the year, but during winter when the pond circulation is even less, it can become a lethal area for your fish. You can reduce the possibility of these anoxic conditions in the dead zones by adding aeration points. If this is not done, then be very aware that these areas can contribute to fish kills during the winter.

It is important to start preparing for winter about a month or two before freeze-up (ice formation). Go through the fish collection and remove fish that are not wanted. Do this in the fall rather than the spring. The goal is to have as low a fish population as possible going into winter.

If there has been any salt added to the koi pond, water changes are needed to significantly reduce the concentration of it. Salt can lower the freezing point of water. So, a pond with salt can become super cooled. I do not have any direct experience with this situation, but I would venture to say that it could be lethal to fish.

Right before ice forms on the pond, the organic matter needs to be removed. If this is done any earlier, leaves can still blow in before it freezes. Once frozen the leaves do not blow into the pond as much. A lot of organic matter is not good for even a summer pond that is being circulated and filtered but this stuff in a pond that is going into winter spells almost certain death to our fish and the ecosystem of the pond.

The outside temperature on the day that this photograph was taken was 25° F. Notice the large hole in the ice. There are three 55-gal plastic drums shown in this picture. They provide structure for the fish to  orient to. Not really necessary for overwintering koi but makes their pond more interesting for them.
The outside temperature on the day that this photograph was taken was 25° F. Notice the large hole in the ice. There are three 55-gal plastic drums shown in this picture. They provide structure for the fish to orient to. Not really necessary for overwintering koi but makes their pond more interesting for them.

By the time winter arrives, most of us have reduced the pond’s water circulation by shutting down high volume water pumps. Biological filtration becomes almost nonexistent due to bacteria going into their restive overwintering stages. These conditions can be lethal with a load of organic matter in the bottom of the pond. A few leaves are okay, but a thick layer of leaves during the winter can be lethal to large koi resting on the bottom next to them. These leaves give off toxic gases and reduce the O2 levels in that area. With heavy aeration this area’s O2 concentration can be increased, but whether it will be enough is in question. So, get that organic matter out of there to ensure the koi’s survival.

Obviously, fish need to enter winter in good vigor and health. Do not add any more fish to a collection for at least a month or two before the onset of cooler conditions. Even though new fish may have been quarantined there is a chance of disease or parasite transmission right before winter. Also, only quality foods should have been fed to the koi all summer and fall. These fish feedings should stop when the water temperatures drop to about 50° F.

With the fish in good shape and the organic load is minimized, the next step is to add aeration. Kloubec Koi Farms owner, Myron Kloubec, uses aeration in his earthen basin ponds. That is how he overwinters his large high quality beautiful koi in East Central Iowa. This is what Myron practices and recommends. That is how I also recommend overwintering ornamental koi ponds under heavy icing conditions.

The important aspects of aeration are to allow O2 to enter the water column and allow toxic gasses to escape. These toxic gases include carbon dioxide, hydrogen sulfide, among others. These gases arise from decomposition of organic matter as well as from fish and other critter respiration and excretions.

The temperature has dropped to about 10° F. The ice is close  to doming over.
The temperature has dropped to about 10° F. The ice is close to doming over.

Adding aeration lowers a pond’s water temperature more quickly than without, but the temperature does not go any lower than what it would have been anyway. The ice in a typical Midwest pond that is 4000-gal and 30˝ deep can be as thick as 15 to 16˝. I have not seen ice thicker than that on this type of pond. Without any aeration the ice forms to within only 15˝ from the pond bottom. That water temperature at the very bottom is as close to the freezing point of water as it can get. By aerating the water you will lower the water temperature of the pond’s water more quickly, but the ice will actually be thinner especially around the area where the bubbles rise. The earth is an incredible heat source. It will continually supply enough heat to successfully overwinter our hardy fish in the Midwest without the pond completely freezing.

A lot of pond owners feel that they need to add a heater of some sort. Some common choices are stock tank heaters that may float or lie on the bottom. Of course, the heating element must be kept away from a rubber liner. The wattage of these can run in the 1500 watt range. This can amount to a lot of dollars to operate, but they do keep a hole in the ice open even in the coldest of conditions. There are smaller ones that use less wattage but the hole that is kept open is much smaller. Some O2 exchange will still occur in these ice free areas that in some years may be enough to overwinter fish. However, for the majority of fish ponds especially koi ponds, it is simply not enough. Circulation provided by aeration is still needed to aid in O2 exchange and to dissipate toxic gases. I cannot emphasize this enough.

Peering down through the one foot deep snow to see the bubbling. In this photo the bubbles cannot be seen due to lighting problems. This aeration point is all the circulation that is needed to overwinter a 10,000-gal pond with 38 large koi. These aeration points must be checked every day to  insure that the air pumps are working.
Peering down through the one foot deep snow to see the bubbling. In this photo the bubbles cannot be seen due to lighting problems. This aeration point is all the circulation that is needed to overwinter a 10,000-gal pond with 38 large koi. These aeration points must be checked every day to insure that the air pumps are working.

Most of the time, I recommend not using any kind of supplemental heat. Lots of energy (and money) is wasted because the heat outside the hole that is maintained is immediately lost to the surrounding air. The heaters do not significantly raise an in-ground pond’s temperature. However, there may be specific instances where the use of supplemental heat is justified. For example, it is important to keep the area around the mouth of a skimmer box ice free if the main water pump, located there, is kept running. However, I generally recommend shutting these large pumps off and removing them as well as any kind of heater.

The best way to add aeration in our ornamental fish ponds is to use heavy duty air pumps. Aquarium air pumps will work okay as long as they are heavy duty and can take cold weather operation. These air pumps only take from 5 to 50 watts of power. I like to protect the air pump from the snow and rain by placing the pump in a 5-gal bucket with a lid on it. Cut a hole in the bottom large enough to allow the cord and air line tubing an exit point. Set the bucket on a couple of bricks so that is off the ground. A brick is placed on top so that the wind will not blow it over. Place the bucket at the side of the pond. Protecting the air pump inside the bucket will also prevent moisture from being pumped down the air line. This moisture will freeze and plug the air line. If this unlikely event should occur just simply abandon the frozen air line and put in another. You might think that the air line will collapse by the ice freezing around it. This does not happen.

Use silicone grade aquarium air line tubing. It stays flexible even during cold conditions. At the end of the air line use a heavy air stone. This needs to be placed in the very center of the pond and on the bottom. In larger ponds more than 20 to 25´ long, it would be wise to create two aeration points with two air stones.

This ice dome has not been pierced yet.
This ice dome has not been pierced yet.

The amount of air that should be supplied to the air line is enough to create what I like to call a “good bubble” – a steady rise of bubbles. I do not like to see a raging boil, which can create currents in the pond. Situations where the fish have to fight a current can be lethal during those long winter months under the ice. They simply do not have that much energy with those kinds of cold conditions. They must be able to simply rest during the winter. After you add the air, watch the fish and see if they are swimming against a current. If so, then reduce the amount of air.

The aeration set-up needs to be checked every day to make sure that it is working. If it fails, for whatever reason, the O2 levels can drop in just a couple of days. If you are going on vacation, then either have an extra air pump running or have someone check what you have while you are gone. The extra air pump is a good idea anyway.

When the outside temperatures are 10 to 15° F or higher there should be an open hole in the ice where the bubbles rise. However, when the temperature drops lower the hole will “dome” over with a thin layer of ice. This is okay since you are pumping air under that dome. O2 is still being supplied to the area and the toxic gases and excess air can escape through micro pores in the dome ice. It is never smart to pound on the ice due to the possibility of shocking the fish but I like to just poke the thin dome ice to make sure that air is still escaping. Leave most of the dome but still poke a small hole. I like to see this dome form, since it keeps a lot of the really cold air from reaching the pond surface.

This is a dome that a hole has been created in.
This is a dome that a hole has been created in.

Some may think that a water pump placed in the center of the pond with the outlet shooting straight up will provide the same benefit. Yes, it will until the intake of the pump plugs or it domes over. In this dome situation the pond surface is sealed off from the atmosphere and there would be no O2 exchange. Both disadvantages are difficult to deal with when you have heavy icing conditions. Use the air pump instead of the water pump.

Shoveling snow off the pond is something I would not do. You do not want to disturb the fish, and by shoveling you are creating quite a disturbance on the ice. You also could get to close to the thin ice near the aeration hole and fall through. Leaving the snow also creates more quiet dark areas in the pond, establishing a better winter environment for the fish. Besides, it can be a lot of work that is not necessary.

Overwintering valuable koi outside under heavy icing conditions is about as stressful to the koi owner as any other fish keeping task. You have to have faith that your aeration technique is going to work and your fish will come out of winter looking a bit thinner but still very healthy.

The other techniques that you can use to overwinter fish are to bring them inside or build a structure over their outdoor pond. Both of these options are a lot of work and expense. Leaving them outside in your properly designed pond, keeping the organic load low, keeping as low a fish population as possible and finally, providing adequate aeration, will ensure that your fish will be fine. These simple techniques discussed here can allow the hobby of keeping koi to prosper in areas where it is very cold.

11 Responses to Overwintering Koi Under Thick Ice

  1. John January 3, 2015 at 1:00 PM #

    I have been overwintering our Koi and 2 red glider turtles for 5 winters in Utah and get an average of 4 to 5 inches of an ice cap. I was mostly concerned about the turtles but they have also wintered for the same 5 years under the ice. I have an aerator that runs 24/7 that keeps a hole in the center of my 2000 gallon pond that is about 4 ft deep. I do not have a heater. I run my circulation pump 2 hours a day and lucky enough that my weir and line from my submerged pump in a deep skimmer drains back so that is does not freeze in the off hours. One thing you mentioned was to dilute the salt content of the water which I have not done and will make sure that happens next fall.

    Great info and thanks for the help!

    • Lora Lee Gelles January 6, 2015 at 6:36 PM #

      From Jamie Beyer:
      Turtles overwinter under the ice in cold climates. They will settle in “muck/leaves” to a depth that is deep enough so that ice will not reach them. Some turtles in our ornamental ponds make the mistake of not getting deep enough and ice will kill them. I also do not like a layer of muck in ponds so the turtles have to settle on the “bare” bottom which is fine. I suspect they will settle among the water lily pots on the bottom. They like the leaves because it disguises them. The most important aspect is that they need oxygen in the water. They will not come up to the surface for air for weeks at a time (until ice out). They absorb enough O2 out of the water during the winter to survive. Their metabolism is extremely slow at all times of the year but in winter it is almost nonexistent — so their demand is extremely low for O2. As an example of their metabolism — their heart rate during the summer is like 10 to 20 beats/minute. During the winter their heart rate I am guessing is 1 beat/2 to 5 minutes.

      So, your depth of 4 feet is good and your aeration is excellent. Lets hope your turtles make the right decision to get to right depth.

      P.S. Frogs overwinter the same way.

      Have fun with water. Jamie

  2. Aaron March 11, 2015 at 12:39 PM #

    Only one thing I see wrong is placing the air stone on the bottom. From my understanding during the cooler winter months the water in the bottom foot or so is 2 to 5 degrees warmer. By putting your air stone directly on the bottom would lif all of that water off the bottom and make it colder

    • Jamie Beyer March 17, 2015 at 10:40 AM #

      Whenever, water is moved by air it draws the water from beneath the aeration point. The water column in that area is circulating up to the surface. Each air bubble, as it rises, creates a small vacuum behind it creating this water movement or circulation. When the bubbles reach the surface, the water movement, that the air created, is still moving and plumes out to the sides of our small water gardens (kind of like a big mushroom of water movement). This water eventually has to go somewhere and so it cascades back down towards the bottom of the pond. In our relatively small ponds this now colder but oxygenated water does reach the bottom. It is displacing the water at the top, with the water from the bottom. Given these physical facts, whether we place an airstone on the bottom or a foot or two off the bottom does not make much difference.

      Here in the Midwest we do have brutal winters and the ice can easily get to 15 to 20 inches in our small water gardens and fish ponds. If the pond is only 36 inches to begin with, then the fish will only have maybe 20 inches of free water to swim around in. Even without aeration the temperature at the very bottom will still be maybe 32.5 degrees F. We are only kidding ourselves if we think we can maintain a warmer environment for our fish on the bottom in our relatively small ponds in really cold climates without adding lots of heat. Adding heaters with enough wattage to actually raise the temperature of our Midwest ponds is really an expensive venture. You can do it but there are better options.

      If you feel better by suspending the airstone off the bottom, then do it. But as I said, it will not make much difference temperature wise. However, if your pond is deep enough and you suspend the airstone several feet off the bottom (only a foot or two down in the water) then you potentially could have a dead zone on the bottom. These are zones of low oxygen levels. The fish suspend themselves just off the bottom and they then could be in a dangerous situation. This potential dangerous situation is eliminated by placing the airstone on the bottom.

      The reason we are placing airstones in the pond to begin with is to eliminate these dead zones. We could also place the airstone at the side of the pond where we are not creating hardly any water movement where the fish are. Believe me this is can be certain death of the ecosystem and the fish. We want good oxygen levels thoughout the pond and good slow circulation does this.

      There are several variables associated with this situation. How heavy is the aeration, size and depth of the pond and how open is the design of the pond? To remove most of these variables just place the aeration on the bottom.

      Have fun water gardening.

  3. Ray March 16, 2015 at 1:24 PM #

    I find your article fascinating and very informative. I am sad to say that I have followed these same techniques for my pond here in southeastern Pennsylvania for many winters – unfortunately this winter as the ice has melted on my pond, I found one of my 17 year old koi floating dead! I was shocked and wondered how could this be? I do have adequate aeration over the winter and I know there is not a build up of debris in my pond going into the winter months. It was brutally cold here this winter and wonder if this was the cause I lost this magnificent fish. I am keeping my fingers crossed that all my other koi have survived. The ice is melting but I still do not have full view of my pond.
    Any suggestions or feedback would be much appreciated. My pond is between 3000 and 4000 gallons and his been here since we constructed it in 1999

    • Jamie Beyer March 17, 2015 at 11:44 AM #

      I am sad to hear of your losing your 17 year old Koi! To attempt to explain the loss of a fish over winter is difficult for me without seeing the fish and doing a necropsy (an autopsy of an animal) on it. I also need to know more information about the pond and what type of winter you experienced.

      However, let me attempt to explain the possible reason you lost of your fish.

      Winter is a tough time for our fish without a doubt. It is critical that our fish go into winter in the best of shape. I am sure that your fish was healthy as far as you know. There are things to happen to our fish in the pond that we may not be aware of. Ingesting something that cannot pass through their intestine and cause a blockage could happen. They could develop an internal tumor that we cannot see. I think you get my point – fish are animals and can contract a condition that we may not know about. Winter arrives and the weakest fish may not survive.

      Losing one fish every now and then is tough to take but it happens to all people that keep fish (or animals in general). However, we still need to go through our steps of making sure that we have done everything right. If we have done everything right and can’t explain it then I say “that’s life”. Now, if you tell me that several fish have died then that red flags that we may more of a problem. That is when we need to be more investigative by doing necropsies on the fish as well as looking at the environmental conditions.

      This winter in Iowa was a brutal one for us. We had extremes that can cause extra stress on our fish. We had a very warm January that allowed our ponds to open up then February came in with a vengeance. These huge fluctuations are tough. Winter is tough as we know but these extremes make it worse. This is where I have become a huge advocate for keeping an ice layer on our ponds as long as we can. My own ponds and my client’s ponds all opened up in January but then a huge snow fell into that open water and plummeted the temperature of our water. It chilled the ponds fast. Stressful.

      Every pond is different. So the important things to remember are not making the critical mistakes that I wrote about in my last article on overwintering.

      Remember to have fun water gardening.

      • Jamie Beyer March 24, 2015 at 12:25 PM #

        I am assuming you were going to empty the pond to clean out the debris. This is the easiest for me and takes the least amount of time.

  4. Catherine March 21, 2015 at 10:48 PM #

    Purchased a property that has a fish pond, I’d guess about 12 x 14 feet+; about 4-5 feet deep in the center. I plan to leave the goldfish out there next winter but my question long before that is…I need to clean out all the leaves from last summer and fall; I know there are frogs in there…When can I scoop out all the leaves and muck w/o doing significant damage to the frogs? I am in Wisconsin, zone 5, the pond has a plastic lining…Much to learn in a very short time!
    Kind regards

    • Jamie Beyer March 24, 2015 at 12:21 PM #

      Good question. I am really impressed by your compassion for all the living things in our ponds — including the frogs.

      It is smart to get the organic matter out of the pond as soon as possible in the spring. You did not mention if there were fish in the pond this winter. I am assuming that there are fish in there.

      So, attempt to save a bunch of the pond water — as much as you can. I have some collapsible pools that can hold 500 gallons that I use when I clean clients ponds. If you can save 25% – great, but more is better. Clean out all the debris without power washing the sides and rocks of the pond. It is okay to power wash the parts of the pond that dried out over winter. The biofilm (I call this the patina of a pond) is dead on the areas that dried. You want to maintain the patina in the rest of the pond that did not dry out. Powerwashing will remove most of this patina if done. Besides it is a lot more work.

      Place the fish, frogs, and any other life that you find in the temporary pool while you clean the pond.

      Now, the critical part is acclimating your critters to the new water conditions that you just refilled the pond with. The original water in ponds that have a huge leaf (organic) load could potentially have a very low pH that the critters have adapted to over winter. Usually this water will be stained with tannins from the decomposing leaves. Water that just came out of the tap or well most likely will be much higher pH. Of course, you have removed the Chlorine but the pH is still relatively high.

      Acclimate the fish (frogs) over the course of a day, add the new pond water to your temporary pool the fish are in. Start out by a 10% increment. If the temporary pool is 500 gallons then take out 50 gallons and replace with your new pond water. Wait an hour or two and repeat with a 20% increment. Wait again and repeat with a larger increment (say 30 to 40%). After you do this 4 or 5 times then your critters are acclimated to your new water garden water. You can pump the water you are taking out of the temporary pool into your pond. You are just exchanging water.

      After 8 to 10 hours of acclimation the critters can then be added into the water garden without the fear of losing them to pH shock. Frogs are not as sensitive to pH shock as fish but I have seen them die because of it. If you have plants in the pond (potted or planted in the bottom) then the frogs will settle around those plants. They like vegetation the best to rest in. If your pond does not have plants then the frogs will simply settle around the rocks. They should be fine.

      Have fun water gardening.

  5. Jamie Beyer January 1, 2015 at 9:41 PM #

    It sounds like you are still running the filtration system in your pond which should provide some aeration. However, I would still run additional aeration but reduce the flow like you thought. If your weather stays below freezing the water temperatures will stay low. I do not know the kinds of fish but if they are Koi or hardy goldfish you will be alright. Please refer to my latest article “You Can’t Make Mistakes”. I would always provide aeration in all four seasons.

    Your comment about flow into your fish pond from an anoxic pond makes me nervous. Since this is the case it makes it even more important to add the additional aeration.

    I assume you have removed most of the organic matter before winter arrived. I wish I knew the kinds, number and size of fish as well as location of where you are at. Size of pond is also good information. If it is a big pond I would think about having two or three aeration points to avoid Dead Zones.

    Have fun ponding. Jamie


  1. ICE - Koi Forum Website - December 29, 2014

    […] ICE Woke up this morning to find a complete covering of ice over the koi pond. The anoxic pond that feeds into it is pretty well free of ice. Being concerned about O2 levels I have now turned on the air supply to the bottom drain in the koi pond – this took about 10min to break up the ice. The temperature about a foot down have dropped from 2.7 pre air to 2.2deg c which I am hoping is a temporary effect. Now I have a dilemma do I leave the air going 24/7 – reduce the air so it is just enough to keep a small hole clear, or replace the aerated BD with a small air stone further up in the pond column – eg by the pond skimmer which is where I am running my filter from over the winter? I found this article on the web and the suggestion seems to be that just reducing the air flow might be the solution – what are your thoughts? Overwintering Koi Under Thick Ice – POND Trade Magazine […]

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