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What are winter fish kills and how can I prevent them? – Decorative Ponds & Water Gardens Q & A


Dyed Pond


Q: What are winter fish kills and how can I prevent them? – Alison in Illinois

Winter Fish Kills, They Don’t Float With Us!
You’ve waited all Winter long for the ice to melt over your water garden so you can run your waterfalls and enjoy your finned friends. Instead, you find your fish floating at the ponds surface, victims of a winter fish kill. What is this phenomenon and how can you prevent it?

Make Some Holes
When a layer of ice forms over the surface of you water garden, it essentially eliminates any transfer of air to or from your pond’s water. What this means to you is that, as debris decompose and your fish consume oxygen, byproducts are produced in the form of gasses that are toxic to your pond’s inhabitants. These gases are trapped under the ice and cannot escape; fresh air from outside the pond cannot reach the water either and so begins the process of the winter fish kill. Keeping a hole in the ice will allow the bad air in the pond to be replenished with good air. Some pond guys and gals use pond De-Icers to maintain an open hole, but many more rely on their aeration systems to do the job.

Pass The Bubbly
We’ve discussed in our past blogs the many benefits of aeration in your water garden. It circulates the water in your pond, infusing it with oxygen which is beneficial to your bacteria and fish. The constant bubbling produced by an aeration system will also keep a hole open in your water garden in the winter months, ensuring the release of those harmful gasses.

Being Supercool is SO Uncool
You have all heard concerns expressed in our past blogs in regards to “supercooling”. While this is a rare occurrence, there are a couple steps you can take to ensure you don’t overdo your winter aeration. When the cold weather comes, move your aeration plates to a shallower part of your pond. This will maintain a warmer layer of water for your fish to retreat to if the water does get a little too chilly. Furthermore, if you have a multiple plate system, you can run your water garden on just one plate for the winter. This will ensure that you have an open hole in your ice and should provide sufficient air supply to your fish as they require less oxygen during these times of decreased activity.

POND TALK: What type of aeration do you use in your pond? How have your fish fared over the past winters?

7 Responses

  1. I, too, keep my pumps going all winter and have never had any problem with winter kills, I have more trouble in the spring when the water starts warming , but have learned that adding beneficial bacteria as soon as the water starts warming helps tremendously.

  2. I’m in MI, leave my falls going all winter and have never lost any fish… knocking on wood!

    • This is the first time in 20 years I lost any fish. Talked to alot of Koi people around here and they also lost alot of koi too. Just seems weird I did nothing different. Em

      • Hi Emily,

        There could have been a lot of reasons for this to happen. Depending on location it was a much tougher winter then in years past. One other common cause may be that if you had aeration in the pond you want to make sure you do not have the stone or plate placed in the deepest part of the pond. This fish will go towards the deepest part of the pond during dormancy and aeration in that portion can cause temperature to drop to low and cause to much water disturbance. If you are using an aerator suspend it from the surface of the pond or place in the shallower portion leaving a calm area for the fish. Another reason may be pond capacity, make sure you do not have to many fish in your pond and a clean healthy bottom without much debris accumulation decomposing in the pond’s bottom. I hope this may give you a few ideas on where to start to protect your fish in future winters.

    • I’m in Michigan also but turned my waterfall off this year (first year having a pond). Do you still have to clean your filters over the winter?
      Thanks.

      • I would recommend doing some cleanout just so the debris is not breaking down in the pond over the winter. I would do a good spring clean out of the filter though when you get ready to start the pond for the season and begin with seasonal defense bacteria treatments.

  3. I kept a hole in my pond and I still had 14 kois die when the ice thawed.

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