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Why Did My Fish Die Over the Winter? – Pond & Lake Q & A

Illustration of No Aeration Versus with Airmax Aeration

Pond & Lake Q & A

Q: I lost all of my fish after the winter. We love to catch fish in the pond and now we have to start over! What happened? And is there anything I can do to prevent this from happening again? – Alfred of Michigan

A: My first thought when I read this question was, “They don’t have an aeration system”. And after speaking with him, come to find out, he didn’t. This is usually always the case during a winter fish kill. Everything seems to be going just fine when all of a sudden one morning you wake up to discover a wave of fish floating on your pond’s surface. This is not a pretty sight, nor is it any fun to clean up. So what causes fish kill and what can you do to prevent it?

What Causes Fish Kill?
During the warmer months of the year a pond with no aeration will contain oxygen towards the surface of the pond. This is because there is an oxygen transfer from air to water at the pond’s surface. The bottom of the pond, however, will contain very little or no oxygen; Certainly not enough to support fish life. Also, the toxins associated with fish waste and other organic biodegradation tend to sink and stay at those lower depths of the pond, polluting the already oxygen-starved water. This unfortunately, condenses your fishes’ habitat area and forces them to live towards the surface of the pond.

There is also a difference in temperature from the bottom of the pond to the surface. The bottom of the pond will be colder than the pond’s surface. The reason for this is because the sun will heat up the surface of the water and since cold water is denser than warmer water, the cold water will fall to the bottom. This difference in temperatures can be quite dramatic at times. Have you ever jumped into a pond and felt the brisk cold water towards your feet? This is the thermocline border. This dramatic change in temperature can cause your fish to stress as they travel from a warm temperature to a cold temperature and back to warm. This stress can lower their immune systems.

During the colder months of the year, the oxygen as well as the thermocline will actually flip. All of a sudden the colder water containing no oxygen will mix with the warmer water with oxygen. As this mixing occurs, the fish are left with few places to go for oxygen and they will eventually suffocate.

Another issue during the winter are toxic gasses. As bottom organics (grass clippings, leaves, trees, twigs, fish waste, etc.) decay, they will create toxic gasses. When ice covers the pond’s surface, these toxic gasses are trapped underneath the ice and will cause a fish kill.

Preventing Fish Kills
Using an Airmax® Aeration System is the single most important way to help prevent winter fish kills. The reasons are simple: With an Airmax® Aeration System, a compressor sits on shore and pumps air down to a diffuser on the pond’s bottom. This air forces the cold water containing no oxygen to the pond’s surface. This water, because it is denser, will fall back to the pond’s bottom. This cycle will repeat and create a convection or current within the water column. This will fill the whole water body with oxygen as well as maintain the same temperature level throughout the pond (see illustration on left).

Also, during the winter months, when ice has covered the surface of the pond. An Airmax® Aeration System will keep a small hole open in the ice to allow those toxic gases to escape.

The Bottom Line: Having aeration will help reduce the chances of fish kill. Also, remember that this is one of many benefits of having an aeration system (Refer to this blog post for the other benefits of aeration).

17 Responses

  1. My Sportsmens club owns a trout and the trout have died for a few years in a row over the winter. We purchased two aerators and the problem was solved for one year. They survived the winter. This spring a checked and there was no trout again. They must have died over the winter again. What should I do.

    • Hi John – Sometimes it is tough to trump Mother Nature no matter how hard you try, and the last few winters have been harsh. If the diffusers are located in the deepest points of the pond try moving them to shallow waters or suspending them closer to the surface for the winter months to allow a calmer area at the pond’s bottom where the fish retreat.

  2. Aeration systems only work in the best of conditions. We have a 3 acre pond with aeration and we just experienced a massive fish kill. High heat and the pond turned suffocating thousands of fish. 3/4 hp four diffuser system for a 3 acre pond should be enough….the experts know nothing.

    • We’re sorry you experienced a fish kill. When used properly aeration systems work in any pond but there are some things you must keep in mind: 1) The depth of your water body dictates how much area a given bottom-diffused system can aerate. The deeper the plate is sunk, the more surface area it can treat. 2) The shape and depth of the pond together dictate how many plates you will need. Ponds that are regular shapes like ovals, rectangles, squares and circles have less bends and curves to block the moving water that an aeration system creates. Ponds with a lot of bends and “fingers” require more plates as isolated areas will need their own plate to effectively move water. 3) You must gradually introduce aeration to prevent water turnovers and fish kills. A pond without aeration stratifies (or forms layers). The bottom cooler layer is not exposed to surface air and is full of gasses, debris and undesired anaerobic bacteria. Simply sinking some aeration plates and firing up the system will instantly mix all of that bottom water together with the the healthier surface water leaving your fish nowhere to retreat to. When installing an aeration system, you want to run the system for as little as 30 minutes the first day and double your run time thereafter (2nd day 1 hour, 3rd day 2 hours, 4th day 4 hours) until you are running the system 24/7. 4) In cases of extreme heat and cold it is ideal to relocate aeration plates so they are not at the absolute bottom of the pond. This will leave a “buffer” of uncirculated water at the bottom of the pond for your fish to retreat to if necessary as the circulated water will become too warm in extreme heat or too cold in extreme cold. This very rarely happens but is not impossible especially with the drought weather North America is experiencing this year. 5)Know your fish. Some fish, like trout, thrive in cool water. If you live in a hot climate and sink your aeration plates to the absolute bottom of your pond you are going to warm the entire water body. If your fish require a specific water condition that is not on par with your climate you will have to make special accommodations in your aeration set-up. In this case you would just want to keep your aeration plates elevated a couple of feet to give your trout an escape to cooler water when needed.

      Aeration systems are astoundingly useful tools, they just have to be used correctly. While the process is mostly straight forward you may always end up with questions or concerns, which is why we always encourage customers to call us when in doubt.

      • I have had a major fish kill this winter and cant figure out why. I have had the pond for 6 years and have never lost a fish. This year has had some major tem swings living hear just out of Chicago. I have an aeration system as well as a heater puck for gas exchange and a portion of the water has always been open. Over the past few weeks my comets have slowly been dying off. The last two days I have noticed my Koi starting to show some signs. Coming to the surface more often, racing aroung, banging into rocks and sitting still between upper rocks almost to take a break from exhaustion. Some of the dead comets were not discovered right away like oters which was apparent from their condition when I pulled them. I had to do a salt treatment for a 16 inch Koi today along with a partial water change 2 days ago maintain a realatively close temp withih 2-4 degrees. The salt treatment seemed to work for abhout a day. I found her today at the very bottom toda, approx. 28 inches in a t shaped 8 inch pipe they normally use for cover. I first thought she died over night and was plucked by a conn but then I spotted some of the tail in the pipe. I flushed her out with a stick and she she swam around for a bit only to go to the bottom and sit normally. One hour later I came to check on her only to find her laying on her sid resting on the rocks. Temp in pond 52, temp in salt bat 52 and the same in my 75 gallon tub where she is now laying on bottom in city water which has very low cholrine. I held her by the tail slowly moving her back and fourth to improve gill function. After doing so she was normal for around 2 -3 min after going back tom her side breathing on the bottom of the tub which I am using a aerator as well. Suggestions to save her asap? John

      • Hi John – I’m sorry to hear of your fish loss. You mentioned the fish running into rocks does it seem like an accident or that they are trying to rub against them? If they are rubbing against them it may indicate that they are trying to get a parasite off. Have you noticed any wounds or spots on the fish? It is possible if some of the dead fish were in there for an extended period of time that it may also be causing additional stress. I would also check the gills for redness or swelling and possibly test the water or do a partial water change on the pond which may help with water quality. The salt bath is the best quick fix treatment if you are not able to identify the direct cause of stress and the more oxygen you can provide the better. You are taking the right steps towards treating the fish but it make take some additional persistence and time for her to recover.

  3. Right now, I cannot afford an aerator system. When I restock my pond do you think knocking a hole in the ice periodically would work?

  4. Kurt,

    Your definitely thinking right. The O2 levels down in that 14ft area are still low and making it inhabitable for any fish, let alone trout. If you can get it down to that 12ft area you may be better off. What type of aeration system do you have?

    • i have a linear air compresse, i think………that i run 2 stones off
      of………………i just find it hard to believe that my water temp down
      that low isn’t in the 60 degree range. i’ve pasted some info on my aeration……..what aerotor can do a good job in that 13 to 14 ft area

      Linear Air Compressor

      Low cost! continuous duty air compressor Runs on 96
      watts very efficient with few moving parts. A great value and will provide
      many years of service.

      • Kurt,

        Your compressor is only 96 watts? That doesn’t sound right. That seems like a very small compressor. Ours are anywhere from 350 to 600 watts.

        Is there a brand name on the compressor. Do you have a link to a website that contains the compressor. Maybe I can take a look at it to see if that is really what you need.

        The other issue you run into is that the water temperature may be 60 degrees but if oxygen levels are too low then the fish won’t go down there.

  5. Hi,
    I have a 1acre pound that is about 14ft deep. I have bass, bluegils, cats and crappies. Last sept. for the heck of it i tried some rainbows. Living in wisc. I figured my water temp. would be ok except in july and aug. might be a problem. I had an algae bloom which i sprayed w/ cutrine plus this spring. Then i read where it couldbe harmful to trout. A month had passed and i saw no dead trout. However last week it was 90 plus here a few days and i’m finding dead trout. Do you think its the hot weather or the cutrine. My pond is only well feed w/ run off….I haven’t ran the well much this spring/summer my water level is staying good. What do you think

    • Kurt,

      To me it sounds like the warm weather. Trout are definitely not a fan of the warmer weather. If it was the Cutrine-Plus is would’ve harmed the trout by the next day. Any copper-based product is not recommended when treating ponds that contain trout, however, if the carbonate hardness of the water is above 50 ppm your trout will actually be fine when treating.

      Hopefully this helps.

      • Thanks for the response. One last thing, with my pond being 14ft deep ifigured it would stay cool enough in the deeper part. Won’t trout find that area. I have 2 aerator stones running at about 8ft because they won’t work pass 10ft. Could the aerators also not be putting enough O2 in the deeper part.


  6. Monika,

    Weedtrine & Cutrine should work, but I would highly recommend using Pondweed Defense instead. Pondweed Defense has no water use restrictions where as Weedtrine has a swimming, irrigation and human/animal consumption restriction for up to 5 days. Also Pondweed is in most cases less expensive as well. When using Pondweed Defense, use 3 quarts Pondweed, 4 oz Cide-Kick and 1 gallon of water. This will treat approximately 2,500 sq. ft. of milfoil.
    Hopefully this helps.

  7. Mark,

    You may have been referring to my post on Super-Cooling, which happens extremely rarely and only will happen in super cold climates such as the US/Canada border and further north. Either way, during the winter, we still suggest to run the aeration system. In the Super-Cooling post though, we talked about how to have aeration running during the winter in those super cold climates. Hope that clears things up!

  8. I have milfoil in my pond. I was told to use Weedtrine-D, Cutrine-Plus and Cide Kick to eliminate this. How much of each and do I spray the surface or inject below the surface? Please advise. Thanks, MONIKA

  9. I thought I read from one of your emails last year to NOT leave my Aeration System on during the winter.

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