All my Large Koi died over winter, Why?


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I use a Pond Breather too. Why did your air stones freeze ?
If it gets cold enough, water will freeze above the stone. The bubbles are still running under there, but ice closes the opening. The stone is 12" below the surface.
The fountain only rises about 8"-10" above the surface, when it's real cold an ice dome forms over it.
 
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Carmine we're almost neighbors, probably 20 miles apart. As for the 13 fish, how big is your pond? To me, that's a lot of fish and if one was sick (you added new ones in the fall) this could have been the problem. Overcrowding, a sick fish and no water changes sounds like a breeding ground for something bad. Add to that the decaying fish floating in the water for 5 months and nothing survived. I would drain the pond and start fresh.
 
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If it gets cold enough, water will freeze above the stone. The bubbles are still running under there, but ice closes the opening. The stone is 12" below the surface.
The fountain only rises about 8"-10" above the surface, when it's real cold an ice dome forms over it.
Gotcha ! I cover the pond for winter, so in my case I don't face the same issue.
 
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I had 6 large koi and 7 smaller ones in the fall. I placed 2 of the donut heaters in the pond, as I do every year. Well, we had a very cold and snowy winter but, they still died this winter. Can some give me some idea on why this happened? Thanks.
I'm very sorry about your fish. My setup is similar to what other people have mentioned. I did not lose any fish despite temps going below -25 F this winter. I run a water pump all winter long. When the piping is covered with snow the water still recirculates under the ice and snow. I also run an airstone and a pond breather. Even when we had a foot of snow on the pond and below zero temperatures the fish were still okay. I have never used a pond heater or covering but my pond is 4 ft deep so the fish are able to go to the bottom where it's warmer. The largest fish need the most amount of dissolved O2 so when something happens to the largest fish and not the smaller ones it's a clear sign of inadequate O2.
 
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@callingcolleen1 has a setup that has been successful for her for many, many years so I won't argue with her conclusions. However, I will tell you, here in Chicago I have visited lots and lots of ponds that are only 24 inches deep. The fish - and most are big koi - stay in the ponds all winter long and they do fine. I think it's important to keep the pond running all winter (which requires some forethought in the design of the pond) and keep your fish load low. Conditions like overcrowding that are hard on fish in the warmer months are made more difficult in the colder months.

And I agree with Colleen that the ice does displace water, but in my experience the ice only gets so thick. So mild winter or really rough winter - the ice cover is basically the same. We experienced the famous polar vortex this year, but the ice was no thicker than it has been in some of our milder winters. Snow cover helps, so we always welcome a nice blanket of snow on the pond, but that's not one you have any control over !
 
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Sorry to here about all the losses this winter. My pond froze solid then thawed multiple times this winter. Second winter with the pond and I left pump running to the falls and the aerator running again. I do move the aerator up and place it on a rock a foot below the surface. I have about 20 Koi and some bullfrogs that all survived thankfully. My fish are still pretty small. Most about 6-10 inches. I'm thinking I will have to thin the herd when they start getting big. I'm pretty conservative with food. I'm in no rush to grow them. This time of year when the water is still on the cold side makes me more nervous then the winter. Last year parasites attacked my fish during the spring. I'm keeping a close eye this year and so far so good.
 

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