Overwintering Koi Under Thick Ice

Published on August 26, 2009

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. 

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.

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

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

3. 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.

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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.
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.

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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.

Aqua UV
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.
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.

36 thoughts on “Overwintering Koi Under Thick Ice”

  1. Pingback: ICE - Koi Forum Website

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      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

  2. 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!

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      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

  3. 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

    1. 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.

  4. 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

    1. 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.

      1. 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.

  5. 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

    1. 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.

  6. We have two small ponds built with liners connected by a ten foot stream. The upper pond is about 2 ft deep and 250+ gallons, the lower is 3.5 ft deep and about 350 gallons. We wintered small comets the first year in the pond. They died. The next year we took our fish out and kept them in the house in a125 gallon fish tank but with the number and size even that is challenging. (We now have a few koi as well as comets. One koi is about 12 inches long). Would an aeration system in our ponds keep them alive over the winter. Thanks

    1. An aeration system is essential to overwintering fish under ice in the Midwest. There are a lot of other factors to consider as well. Please refer to all of my articles on this subject.

  7. After reading the “Overwintering” article I felt compelled to write my thoughts.

    I’m in south-central Wisconsin. I have a pond that’s about 6,000 gallons and at the one spot, (the deepest spot), it is about 36 inches deep.

    We have left out Koi in the pond since we began 10 or 12 years ago, and have rarely lost any fish.

    Right or wrong, here’s what we do.

    As temperatures approach heavy freezing level, we remove the above ground pump that circulates our water from the pond to our “filter”. The filter is simply a large wooden box that houses lava rock. It ‘upfills’ and overflows into a stream the goes back into the pond.

    We remove the pump and drain the ‘filter’ box. It’s at this point when we remove as much ‘gunk’ as possible from the filter box.

    Now you must realize that we are fortunate that our surrounding trees are evergreens, (cedars), and therefore we do not have much of a leaf problem.

    It’s at this point where I “lace” together two plastic milk crates, (using plastic tie straps), and “lace” a very small submersible pump onto the top of the apparatus fixing it so the discharge water stream shoots straight up in the air.

    Then I locate this in that deep area of the pond moving it about so that it’s shooting a stream of water which is about as big around as your thumb, ( the size of the output stream will depend on the pump you acquire). It’s all located so that this stream shoots straight up into the air, and typically that is about 4 to 6 inches.

    The movement of the water falling back from the pump will keep the immediate area open from freezing UNTIL the temperature drops into the zero degree and below area.

    Ice will form and freeze closer and closer to this spot where the water is shooting up and falling back into the pond. Typically the ice will form so as to leave an open spot that’s about 8 inches to 12 inches in diameter. As the temperature gets colder the ice will form closer and closer. As the temperature increases the open hole will get a bit larger.

    When the temperature gets REALLY cold a dome of ice will begin to build up around the output pipe and stream of output water.

    I have closely examined this to try to figure out how and why this occurs. As a drop of water falls back into the pond it causes a number of smaller droplets to be splashed back up into the air. Some of these droplets will fall on the edge of the ice and as they do eventually they build up and this little dome begins to form.

    If the temperature is cold enough for a long enough period of time it will eventually form right up over the top and when that happens I go out every couple of days and just tap the very top of the dome to break the ice and allow any gases to escape.

    Years ago, before we even had completed our pond, we had a few Koi in a cattle tank in our garage. The temps. got cold but did not freeze the water at all. I would look down into the tank to see these fish to be pretty much ‘suspended’. That is they basically just ‘hung there’. Of course we were NOT feeding them.

    One day I accidentally happened to lightly kick the edge of the metal tank and that vibration through the water caused the fish to ‘react’, that is they quickly moved for a second or two.

    I have always told others that the fish ‘hibernate’. I’m sure that’s not the right word, but as I explain that as the metabolism of other cold blooded animals caused them to pretty much go into a ‘suspended animation’ stage, so to do these fish.

    I tried various heating devices but soon arrived at my small submersible, inexpensive, economical pump and it works very well for me.

    I’m not using it to aerate the pond, there doesn’t seem to be a need to do so, (especially since the fish are using very little oxygen in their ‘hibernation’ phase), beyond the oxygen generated by the water falling back into the pond. I’m using it to allow for any potentially dangerous gases to escape.

  8. Do you think it is at all possible to overwinter fish in Manitoba if I use a heater. Ok here is the hard part winters can be -10 to -15 F. Maybe this is just a crazy idea.

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      Yes. The temperature during the winters in the Midwest of the United States can easily fall to -15 to -20 F. Hardy pond fish (goldfish and Koi) are overwintered in these conditions as long as you adhere to the advice given in my article. Have fun water gardening. Jamie

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      Yes. The temperature during the winters in the Midwest of the United States can easily fall to -15 to -20 F. Hardy pond fish (goldfish and Koi) are overwintered in these conditions as long as you adhere to the advice given in my article. Have fun water gardening. Jamie

  9. I have a outdoor pond 22 feet deep and measures 50 feet x50 feet. We lost about 90 percent of the koi last year and would like suggestions on how to prevent. We have no power source to run aeration pumps. I plan on putting three large bales of barley straw into the pond in the hopes of the pond not freezing completely over. Any recommendations would be appreciated. The pond currently has approximately 200 koi.

  10. Quick question… would putting koi in a small lake in central Ohio be a bad idea? Or would it be better to have a super controlled, smaller area? Reading all the way down to the bottom, I’d imagine something like a lake would be ok during winter, though I’m wondering it it might be too much.

  11. Great article I’m glad I read it as I was just about to add salt to the pond as was suggested to me (1-2 lbs/100 gallons). I have an approximate 10 000 gallon pond with 13 6-8″ Koi, a mix of a dozen Shubunkins and Sarasa Comets also about 6-8″. The goldfish spawned numerous times this past summer so there are also 60+ babies of varying sizes from 1 – 2 inches. I have a vacuum but it quit on me so I still have a light layer of leaves that have settled at the bottom right in the middle section of the pond where I have been unable to scoop them out as I can’t reach. We have just had a thaw so the thin ice layer is off the pond again and weather looks on the warm side (0 to 5 C day time temps) right through December… should I either rent a pond vacuum or buy a new one and make sure that all of the leaves are gone. I worry now that I may disturb frogs, my one painted turtle (I hope he’s down there), and the fish which are all hanging around at the bottom where the leaves are now.

    The Koi are only domestic so I don’t have a lot invested in them but I have grown very attached as I taught them to eat out of my hand this summer and would hate to lose them. I notice from a previous poster that you like to know location. I live in Southern Ontario about 1.5 hours N of Toronto. This will be my ponds 2nd winter. Last year was a frigidly cold winter up here with -30C temps most of February… I only had about 7 cheap pet store Shubunkins about 3 inches that over wintered and without aeration they survived. I didn’t know about cleaning the leaves out of the pond prior to winter so it didn’t get done until the Spring. I now have a good size aerator with a 10″ round air disk that I will move this weekend to the centre of the pond – again advice I was given was to have it at the edge about 2 feet deep so as not to create a current in the middle and have a heater above the aerator so that I didn’t super cool the pond. Luckily I hadn’t bought the heater or salt yet!! Your advice would be greatly appreciated!!

    1. Avatar photo

      I like most of the leaves (organic matter) out of the pond before winter. A light layer of leaves is okay as long as you do not have a “wad” of leaves in areas of the pond. These areas can create anaerobic conditions that can kill critters like fish that need oxygen. However, with aeration, the gases that anaerobic conditions creates is dissipated. So I think you are okay with the situation that you have described as long as you keep your aerator working ALL winter. Check on it every day or as often as you physically can.

      I have found that the fish do not like to be right next to the aeration point even though the temperature is the same throughout the entire pond. They simply do not like to fight a current set up by the aeration. So do not have too heavy of aeration. A raging boil is too much. I like a simple bubble coming from my aeration points. The fish will be calmer and stay snug under that ice.

      Your description of the 10 inch round air disk with a good size aerator sounds like too heavy aeration. Remember to keep it to a simple bubble.

      Have fun water gardening. Jamie

  12. Hi Jamie,

    I have read a number of your articles and very much appreciate the insights given. I live in Conquerall Mills, Nova Scotia and our winters can be tough but probably not like Iowa. Coldest normally say -20C or about 0 F. This summer I built a water garden, two connected pools about 5′ 6″ deep. I lined the pool with clay, did a less than perfect job knowing i would have some leakage but over time it would hopefully get better. The two pools are about 16′ and 10′ in diameter and I think the total volume is in the 15,000 gallon range. I constructed it with clay on the bottom and lined it with large rocks and boulders, back filling with clay behind the rocks as I progressed. I can send you pictures if you let me know how to do that.

    When I filled the pond with water I learned that I was loosing about 12″ a day. The water table was quite low and we have had a very dry summer. I decided to to a polymer sealing treatment and that cut the leakage to about 1″ a day which I found very acceptable. So I put 14 koi in the pond and they have done fabulously. But my leakage has increased to about 6″ a day.

    In anticipation of leakage I constructed another pond which is lower in the water table intending to make that my supply source for keeping the water level where I wanted it. It has worked well and my pond level is constant, being topped up 4 or 5 times a day.

    With winter coming on I have become quite concerned about the effect of topping up the level during the winter mostly in terms of the effect on the Koi. Also, the water availability in winter is very questionable. So my worry is that the pond level will drop to a point where the koi will die because the water level drops so low that they will freeze.

    So essentially, this year, I have decided to bring them inside and do the best I can. I appreciated your article on that topic.

    I appreciate any advice you might have.

    1. Avatar photo

      From Jamie Beyer:
      Your question brings up a very good point that I have not addressed very well in my articles. It is a very good idea to make water changes during the winter in our ponds with a big fish population. How many and how much each time is the next question. The number of water changes really depends on the number of fish overwintered. I would do one water change at the end of January for sure and you could go as high as to once/month if needed with more fish overwintered. I would do a 10 to 15 percent change each time. Of course, you will need to remove Chlorine if it’s in the water.

      Now, to answer your pond leakage question with a clay lined pond. The one inch/day leak is acceptable as far as the need to make water changes anyway. A larger leak means you are making much larger water changes, of course. You can do it but it can get expensive and you can’t go on vacation unless you have a pondsitter. One winter I had a client that had a 500 gallon/day leak in a 10,000 gallon pond. We filled that pond everyday with city tap water. It was expensive but the fish overwintered just fine. As a side note the client had such a high water bill that the city called him asking if there was a problem.

      If you do not access to a water source during the winter then that is an issue you will need to resolve or move your fish indoors. Your fish need the maximum volume that your pond will hold to overwinter successfully.

      I am not sure the reason you used clay instead of the quality liners that we have for ponderers these days. The expense would be similar especially when you factor in the cost of water for even a slow leaking pond over time. However, the clay liner is your choice and I will not be critical of your choice as long as the fish are healthy.

      Good luck and have fun water gardening. Jamie

  13. Three years ago my wife and I moved from Chicago to a 10 acre mostly wooded property in Indiana. The house is high above the road on a steep hill. We get a lot of rain and there is a lot of run off towards the bottom. The general area we are in is considered a flood zone, and I thought that the bottom of the hill would be a great place to put a pond.

    I am originally from Hawaii where there is a heavy influence of Japanese culture. I was pleased to see your article and find that koi can actually survive the winters here.

    I have been planning on making a hidden japanese style rock garden at the end of a trail through the woods at the bottom of the hill. It would be a nice place to relax and meditate and it would be a comfortable retreat that reminds me of my home in Hawaii.

    Since koi can survive the winters here, I would like to incorporate a pond into this rock garden retreat. It would be the perfect addition to my plan. I have two problems.

    1. There is no power to the area and it would be extremely expensive to run a line there because it is so far from the house and the power box. Would a solar powered aerator be sufficient for a pond of about 10,000 – 15,000 gallons in size? Are solar aerators reliable?

    2. I have been reading that organic material can be lethal. Because the area in question is in the middle of a forest of mostly oak with some walnut and other species of trees, there is always a tremendous amount of leaves to deal with in the Fall. It would be impossible to get it all out and keep it free of organic material. How critical is it to get it all out? Are there ways around this? How do wild fish survive this problem?

    Thank you for a wonderfully informative article which has given me a little hope that I may be able to pull this off.

    PS: If Japanese koi could not survive my specific situation, do you have any alternate suggestions for hardier fish that would be more appropriate?

    Thanks again.

    1. Avatar photo
      Lora Lee Gelles

      Your email generates a lot of thoughts that you need to think about before you build the pond. You are very smart asking the questions BEFORE you build.

      First of all, I would not normally build a pond in an area that can flood. It is amazing how often big rain events are occurring in the last 20 years. Find out when the last flood was where you are thinking of building the pond, then consider the options. One option is to build a berm around the pond so when the area floods, your berm protects the pond.

      Twenty five years ago I build a couple ponds that were out in the “back 40” as they say in Iowa. Both were between 100 and 200 yards from the house. So…. to appreciate them you needed to make the “trip”. In one instance we put in a gazebo to enhance the living area next to the pond, the other we put in a nice path to the pond. Both ponds became neglected after only a few years. Why? Out of sight — out of mind. When you do not see them every day — which is vital for a Koi pond — you eventually do not maintain them. There are exceptions but 99 times out of a hundred I would say that this is true from my experience. A Koi pond is even more vital to check everyday since you may have more filtration and more expensive fish. I always say to build these things “up close and personal”. Why would you invest this much in something that you only will visit occasionally? So think about this when you build a distance from the house.

      Solar power options for a Koi pond are extremely limited unless you have a large solar panel (expensive). Solar powered pumps have come a long ways but it would be so expensive to get the right sized pump with enough batteries and solar panels. I am not an expert on this stuff but I have not seen anything that would come close to what is needed at a reasonable cost.

      Organic material that accumulates in an aquatic system uses up a lot of oxygen as it decomposes. Some organic matter is okay and can be healthy since the fish like to “grub” or graze in it for food. However, a layer of more than an inch or so can become toxic in our relatively small ponds. It depends on the amount of circulation and filtration.

      You would be inviting disaster conditions without the removal of this organic matter especially with a solar powered filter, which means small in my mind.

      Natural fish populations would also die in the above conditions. The reason they can survive in earthen basin ponds are many. The ponds are bigger and should be deep so they have volume. That is a key component. Shallow ponds of less than 8 feet in the Midwest can be lethal (in most winters) to a fish population without added aeration.

      Another reason they can survive is that some natural circulation occurs in large lakes and ponds by wind and an inflow of water.

      Earthen basin ponds also have an interesting ecosystem between the earth and the organic matter that is in contact with it It is an area of limnology that I do not know very well and most limnologists do not know the entire interaction in this zone. Suffice it to say that this ecosystem, which is mostly anaerobic (without oxygen) will contribute to the survival of the fish that I do not understand — I just know that it exists.

      Finally, most earthen basin ponds here in the Midwest that are located in heavy timber do not have healthy fish populations — unless they are large and/or they are aerated by man. I am generalizing some when I say this but this is my experience.

      Finally, you asked if there are more hardy fish that can survive in the conditions you have spelled out? There are some species of green sunfish that can survive in lower oxygen, and maybe some kinds of chubs. Not good options for an ornamental pond. But even those species need decent oxygen conditions. One thing that I am always talking about is to consider the entire ecosystem when evaluating options. This needs to be a major part of the focus and not just on the fish. The critters in the biofilms (patina) and in the water column are critical for a healthy ecosystem. Even amoebas, rotifers and paramecium need good oxygen levels.

      Have fun ponding. Jamie

  14. Concerned about koi bio-load if overwintering in Minnesota backyard pond.
    I read several articles with info re overwintering koi in a pond, but I still have some concerns and I’m hoping you can help.

    40 inches at the deepest part, that bottom area is about 24×24;
    the next level is about 18-24” deep with an area of 18×30;
    the remainder varies from 18 to 10.

    2 – 20” long females and 1 – 16” male (they are the original 3 we purchased when they were only 5”).
    18 – 10” to 16” (from the original three breeding)

    Past over-wintering
    The past 3 winters we brought them in the garage in 2 large stock tanks, covered the tanks and don’t feed them. Each tank has an individual canister filter with a light bulb near the tubes to keep them flowing. We still have all the supplies for over-wintering in the garage and handling/moving them isn’t a problem, although we’ve never waited this late to move them.
    This winter we’re thinking about overwintering them in the pond. We’ve purchased 2 surface heaters and a good quality aerator. We cleaned as many leaves as we could out of the pond. We also bought a pond vac to get the bottom cleared.

    I’m just not sure that there’s enough space at the deepest part of the pond for all the fish. It seems that they’ll be really packed in there. I know there’s no guarantee, but I’m wondering what your opinion is.


    1. Amy

      You are right to be concerned about having too many fish. Since you have never overwintered your fish in the pond before it would be a big step for you to overwinter them in the pond.

      You do need to trim your population of fish but this time of the year is not good since no one wants to add fish to a pond that is going dormant.

      Since the pond is rather small and you have a large population I would hedge my bets and overwinter some of them in the pond and some of them in the garage. You have been successful with the inside technique so I would take your original larger fish inside. You could also add a couple of other fish that you like to the inside as well. The rest of the fish leave in the pond and use the techniques in my article to overwinter them. Sounds like you are doing a good job removing most of the organic matter and sediment.

      This way you will gain confidence in overwintering them in the pond. A year from now , after you trim the population next spring, you will have the necessary confidence and experience to overwinter all your fish outside.

      Have fun water gardening.

  15. Great article. I learned a few new things. I may rethink how i do things, but I have a small pond here in Ontario and relatively small Koi. Here is how I manage the overwinter issue. In the deepest part of the pond I place a large round metal duct with a hole cut in the side to permit some water exchange ( and the smaller Koi/Goldfish can enter). I use a floating cattle tank heater that fits inside of the duct that keeps this area ice free. It only comes on when it senses the water freezing and only has a very small section that it needs to keep ice free. I then stack round pieces of rigid insulation to the top of the pipe (with a small groove for the power cable). I then cap the open top with a large flower pot to keep the elements out and further minimize heat loss due to wind. I also have some oxygen plants which I had been told continue to produce O2 even under ice, so this section I shovel the snow to maximize daylight. Seems to work, no Koi losses for the last few years. I also added a small LED light by the pond that confirms the power is on (my one time I lost fish due to a tripped circuit.) No huge change in electricity that I can detect or cry about.

  16. I have a large swimming pond in Vermont and we get some very cold winters. It is not practical, due to the size of the pond, to clean leaf matter off the pond. I try, but in fall, with leaves and needles from Hemlock trees its not possible to remove them all. Plus the pond is old, its been around for some 30+ years and maintained by a small but steady spring (somewhere) as the water level hardly changes, even during drier times. It brims with life, from frogs, salamanders, some small minnows and even wild ducks.

    Question 1.Will I be able to add Koi to this pond if I aerate the pond. Pond size is approximately 100m by 50m and depth is 8 feet+ It does freeze over solidly during the winter.

    Question 2 where should I place the aerator. It would be risky to place it in the middle as I would probably not be able to get there safely to pierce the ice dome – what would you recommend for large swimming ponds.


    1. Good questions.

      With this size and depth of pond it is more likely that you will be able to overwinter Koi in it without removing all of the organic matter. There are a lot of Koi people out there that say that you cannot even reliably overwinter Koi even when all of the organic matter is removed. I have done it for decades and I say you can as long as you do the steps outlined in the article including removing the organics. If you skip any of the steps then the reliability is reduced. With the size of pond that you talking about, you may be successful. I wish I could be more confident. I have seen where Koi will spend most of the winter on top of a pile of rotting leaves. Some of these Koi developed a fungus on the ventral side of the fish. They died due to the fungus growing into the gills.

      For question 2, I would have two or three aeration points in this big pond. As long as you make sure that the air is still being pumped under the ice then the dome of ice does not need to be pierced. In the last 10 years I have been recommending to add two air lines to each aeration point. That way you are more sure that air is being pumped to that point. Each aeration point with each having two air lines.

      Have fun water gardening. Jamie

  17. I need some big pond/harsh winter Advice PLEASE.
    Last winter was my first attempt to overwinter koi – in hindsight I made lots of mistakes. I left a lot of organic matter and fallen leaves in the pond, and had a single aerator that was about 10” deep off to an edge of the pond. 2 of 40 koi survived. Many were developing fungal patches as the ice was forming.

    My pond is ~20,000 gallons. It is round, and natural. Mud/clay bottom, with some central vegetation, about 40’ by 40’, and 5’ deep in the centre. The pond is fed by a year-round underground spring that brings cold water in, to drain out the opposite side. I live in calgary Canada. We have winters that sound similar to yours – lows in the -20 and -30’s Celsius. 2-3ft of snow is the norm. And our winters are long – October to May.

    Trying to learn from my mistakes, I have learned the signs of fish disease and have treated promptly. I have raked out a significant portion of the organic matter, and will continue to do so as leaves fall. I have added six aeration disks, on heavy duty lines and pumps. I tested this brand last winter with a single line/disk, and it worked all winter. However, this year I’ve added 5 additional disks and moved them more towards the center. Currently they circle around the center at a depth of about 3-4’. I’ve had pond enthusiasts tell me that my plan will disrupt thermal layering significantly and that I’ll be pulling cold air down to “warm” water depths. Your site is the first I’ve come across that seems to support my approach. Because the spring water is so cold, year round, the warmest my pond gets in the hottest summer days is about 12-13 degrees (Celsius). We’ve just started transitioning into fall and my pond is down to 6-8 degrees. All the small koi have disappeared into the vegetation to hibernate, and my five large koi (14”+) are parked on the bottom side-by-side. I feel having the aerators at a close to bottom depth will help facilitate deep layer gas exchange, as you’ve mentioned, but also hopefully help keep a bottom layer that stays liquid. With the spring water running through, and following the path of least resistance (ie ice), my hope is that it will run centrally where my aerators are, and deeply across the bottom where hopefully the bottom layer is not frozen.

    I’d love your input – if there is a way I could send a few pond pictures or a video of my setup – your expertise and advice would be hugely appreciated!!!

    1. Hi Jennifer

      There is a variable in your question that is difficult for me to evaluate. That is the input of your underground spring that runs into your pond. Underground water has no oxygen in it and it is normally “cold”. The temperature is usually very constant and at the temperature of the deep substrate. This temperature in Iowa is around 50 degrees F. The spring water temperature would moderate your winter ponds temperature. How much is dependent on the amount of flow of your spring. I am guessing that your normal flow is not that much but it will influence your pond temperature — both winter and summer.

      I am very nervous whenever a client tells me that they have a spring that will fill their pond. If you have a pond with trout in it — then I am good with having a spring. Since trout are a cold water fish they benefit from these temps. However, Koi are not a cold water fish. They can tolerate cold water for a period of time but they need the warmth of summer pond temperatures for a lot of reasons that I will not get into.

      Also, I mentioned that spring water is normally low on oxygen — this is not good for your pond’s ecosystem (including your fish) especially during the winter. Your heavy aeration definitely helps during the summer but during the winter heavy aeration is not good. Yes to aeration, like I’ve mentioned in my articles, but not heavy aeration. What is heavy aeration? A “slow bubble” is all you need during the winter. You do not want your fish fighting a current. I like to say it is difficult for them to “flick a fin” in very cold water and when they have to fight a current — this is something they will avoid — if they can. Ponds that have a lot of heavy aeration points will make it difficult for the Koi to find a spot near the bottom that is “comfortable” for them. In situations like this the fish will not move from a spot and they simply lie on the bottom motionless for long periods of time. You mentioned that your Koi are side by side on the bottom “parked”. This is an obvious sign to me that they are not moving around much. When fish lay on top of rotting organic matter for a long period of time — fungus can develop on their ventral side and into their gills. It is vital that your fish swim around slowly in the pond. Also, not having a lot of organic matter is vital to the health of your fish.

      In Koi ponds it is rare for me to see Koi “resting” near aeration points. You have to have the aeration but do so only with a slow bubble at each aeration point. In your pond, it sounds to me like you need at least two aeration points but only with a slow bubble at each. You do not need aeration disks – this creates more turbulence and current. I like to use an open air line that has a weight on it to keep it at a spot. It is also vital to make sure these are working every day all winter. I want to emphasize every day. Even a few days of no aeration can be lethal depending on organic load. I will use two aerators in situations where I know the client cannot check their pond every day — it is that vital.

      I also like to make sure the aeration points are towards the center of the pond. Aerating at the side will create good oxygen in that area but there could be “dead zones” in other areas of the pond. These are areas without oxygen and are lethal to the fish. Please read my article on dead zones.

      To address the aspect of thermal layering — this is a subject that I need to address in more detail in a separate article.

      I am still nervous about your spring — I wish I could be more confident in my answer to you about this variable.

      Have fun water gardening. Jamie

      1. Hi Jamie,

        Thanks for your reply. The spring water comes out above ground and then travels down a rock slide/waterfall (approximately 100 feet of passing over rocks with lots of turbulence) before being funneled over a small rock waterfall into the pond. So I imagine it gets a bit of aeration during the passage. But yes, it is cold year-round (about 8 degrees Celsius). My pond is sitting at about 6 degrees now, the waterfall/spring and aeration disks do create some water movement, but there is no current. The fish can “park” pretty much anywhere without fighting a current. There is water movement, but not enough they have to work against it. They do spend most their time in the center on the bottom now that it’s cooler, but still swim around. They are still eating actually, but I’ve switched them to wheat germ feed. Just much less active than when it is warmer. By heavy aeration – I have six air disks. They all bubble, but there are no pumps/artificial water movement etc. They are strictly air disks. We will see how the winter goes…

  18. I am considering building a small waterfall/pond in my yard, about 20 feet from the edge of a very, very large lake in Arkansas. My intention is to pump water from the lake to the top of the waterfall, then allow it to run down the waterfall into the small pond (maybe 5 ft x 8 ft, 3 ft deep) and then over a kind of spillway and down a stream and back into the lake. I know nothing about any of this other than I have imagined in in my minds eye for more than a decade and only just now have a house on a lake where I can do this.
    Is this a decent candidate for a small koi population? Does this require any special considerations? Is there any reason why this might be a bad idea?

    1. Hi Jessie

      Your idea of using lake water in a Koi pond is done routinely in the Midwest. As long as the lake is not polluted the water should be okay. Just what is polluted? When it comes to high quality Koi even a small amount of pollutants is not acceptable. The water may be good quality one day but the next something may have entered the lake that caused the quality to be compromised. Could be someone spraying a field with something and a heavy rain caused whatever chemical to wash into the lake. The amount of contamination is dependent on several variables. Amount of water in the lake (how big is it) and how heavy the rainfall are just two examples of variables. There are many more but the point is that you cannot control this. If the lake is really large then, as they say, “the solution to pollution is dilution”.

      I do have another concern. Parasites/disease in the native fish of the lake can infest your fish. Parasites will be prevalent. Things like anchor worm is common in Midwestern lakes. Then there are the smaller parasites that will show up. You may be treating for these on a regular basis. The key word here is “may”. The chances are very high — I think.

      The other concern is the amount of sediment in the lake water. If it has a high sediment load, then this will end up in your water garden. This dependent on how you draw the water from the lake. Both this concern and the parasites/disease can be mostly controlled by using two kinds of filters. UV filters combined with mechanical filters that filter out fine sediment may work for you. Sediment may not be an issue in the lake you are talking about but the parasites will be.

      These are my initial thoughts on this. I would not invest a lot of money in expensive fish until you have tried it in your situation. Sounds like a challenge to me but one that may be worthwhile. One last thought — read my article on mink predation. Being right next to a large body of naturally occurring water will always have a mink population. This has nothing to do with you drawing water out of the lake to use in your water garden but it is a concern that I would think about.

      Have fun water gardening.

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