Lost my largest koi with a water change.


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Hi everyone, my first post here.

My pond water (6 by 6 and 3 ft deep on one end) was getting thick with algae, I did a partial water change last week, a little more than half total, the fish still seemed lethargic and were wanting to stay near the surface under the waterfall, and water clarity kept getting worse. Yesterday I did an almost complete water change, keeping the fish in the pond with 6 in of water on the bottom. I rinsed out sediment that was in the rocks and kept water flowing as my pump pushed water out at the same time. This was kicking up some chunks of decayed material, but did this gradually and got most of the debris out. I then removed the pump and started slowly filling, and added stress coat and beneficial bacteria as it was filling. The fish seemed to enjoy the turbulent water, and they were swimming around after it was completely filled, but maybe a little lethargic. This morning I found my 13 inch 3 year old koi at the bottom of the pond. The other seem to be in okay shape, 3 10 to 12 inch koi and a dozen goldfish. This has a UV filter, this didn't keep up with the warm weather bloom in North Carolina. Do you think it was stress from temperature change, too much sediment in the transition, or actually removing too much bacteria and original pond water, that caused the death of my biggest koi?
 

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If they linger by the falls, they probably are gasping for oxygen. An aerator might help.
But I hate to tell you this.....
Your pond is way too small for all those koi and they are competing for the oxygen. Koi need a lot of space and they continually grow and that's probably what happened. They got too big.
It's also possible your ammonia levels got too high from the large fish load. Do you have a good test kit?

Your algae bloom is most likely caused by the large fish load, there's too many nutrients (from the fish waste). The algae feed off of the excess nutrients. Lots of plants can help starve the algae of nutrients. But the root of the problem is your fish load.
Time to build a bigger pond!

This may be a dumb question on my part, but Is there chlorine in your tap water? If so, you need to add a dechlorinizer.
 
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Sorry for the loss of your koi. That many fish in 6 inches of water.....low O2, high ammonia, stress and possible disturbed sediments / toxins.......could be any of those things. Also, with a pond of basically new water, the PH could have been different than their pond water.

I agree with @poconojoe your fish load is too high for the space and you're probably under filtered.
 

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@twinwildwolf Sorry for your fish loss.
Start diggin cuz you are gonna need a much bigger pond for all those fish. Koi get huge!
 
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Welcome @twinwildwolf - sorry that it's pond trouble that has you sharing your first post.

As others have said it's likely your fish got too large for your small pond. You have about 800 gallons if your measurements are correct, which is a good size for goldfish. "Warm weather bloom" is only a thing if you have an excess of nutrients in the pond caused by too many fish and not enough plants. A UV filter cannot correct that issue.
 
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Thanks everybody for your input! Yes, these fish have been with me for the most part for 2 to 3 years. I imagine I stressed the large fish out as it was relatively sudden after the water change. I do have test strips and the ammonia levels are generally low. The plants are usually covering half of them, I have to weed them out from time to time as they multiply quickly. I heard that too many plants are actually an oxygen drain, is this true?

I'm not sure if I want to commit to a bigger pond, I imagine I would have to take over more of my yard, one thing I left out on my dimensions is about one-third of it has a plateau at 1 feet deep by the pump well. So I imagine it is smaller in volume then most of you figured. I may have to stick to goldfish, or find a place to rehome the koi once they get large.
 
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I would not wait to find a new home for your koi - you're way overstocked. Those other fish are already too big and will meet the same fate I'm afraid. All that algae that you're fighting is actually keeping your fish alive. It's concerning when you say "ammonia levels are generally low" - ammonia levels need to be zero and especially in an established pond. If you are seeing any ammonia at all, you need to address that ASAP.

And I know this is not the news anyone wants to hear, but unfortunately we see it time and time again. Koi do not belong in these small ponds. We buy them small and they are so pretty, but they grow very, very fast. I wish fish sellers were more responsible about who they sell koi to - there should be some guarantee that these fish are going into adequate sized ponds. Even if you boost the filtration to the point that you could maintain the water quality, a koi can get three feet long - yours will barely be able to turn around in your pond, let alone swim and get the exercise they need.

And please know I'm not blaming you - like I said, people who sell koi should do a better job of educating the buyer. I see these for sale in aquariums and it makes me so sad. Our local pet store sells them from a stock tank in a separate area away from the aquarium fish and they have signs posted detailing what kind of environment koi require with photos of full grown koi.
 
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