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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?

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).

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