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Dr. B.
Dr. B., Veterinarian
Category: Pet
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Experience:  I am a small animal veterinarian and am happy to discuss any concerns & questions you have on any species.
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I am a koi novice. We bought a home with a large koi pond --

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I am a koi novice. We bought a home with a large koi pond -- 32 Koi (the majority are 4 years old, with several babies). I hired "experts" to do an annual deep cleaning yesterday. They completely emptied the pond, greatly thinned out the plant life, added the fish back in to a couple feet of new (dechlorinated) water (with the 2 garden hoses still filling the pond. After several hours, several fish are floating on their side, struggling to breath. What is going on?

Hello, I am Dr. B, a licensed veterinarian and I would like to help you with you today.



When you say they were struggling to breathe, were they air gulping at the surface or just showing elevation in their respiratory rate?


When they are on their side, can then return to a normal position + swim if prompted?


Is the water from the hose the same temperature as the pond water?

Am I right to assume only the first few feet of water was treated?


You have noted that the pumps were off (since you turned them back on). But was there any other oxygen source (ie aerator, fountain, waterfall, etc) for the pond?



Since you appear to be currently offline, I will leave my thoughts here.

The signs you have described are all very suggestive of stress + oxygen deprivation for the fish. We can see similar signs with nitrite or nitrate poisoning; but if the pond has just been cleaned out this would be less likely. As well, I assume that the pond cleaners didn’t use any cleanser that may have left a toxic residue here (and hopefully the untreated hose water is not grossly abnormal).

Now stress in fish can arise from any sudden change in their environment. If the water is completely new, then this can be the source of the problem. We can see problems with total water changes due to temperature fluctuations, pH shock, and water hardness alternations. Koi are particularly sensitive to rapid temperature and pH changes. As well, we can even have situations where the previous pond water was terribly toxic (with high nitrates/nitrites/ammonia); yet when we change it to new water we end up with fish that cannot cope with the dramatic change. This is why in nitrogenous waste toxicity issues, we have to do gradual water changes to wean them down to normal, clean water without inducing fatal shock to their systems.

As well, we need to consider the oxygenation ability of these fish, especially if you think they are struggling to breathe. While fresh water should have a good oxygen supply, it is possible that the stress of change has led to the fish having an elevation in respiration and this then lowers the oxygen resources of the water. As well, if the stocking density in this pond was at maximum for the pond’s oxygen carrying capacity (ie ideal water volume is 100 - 150 gallons per small koi (2" - 8")), we could see this problem be secondary to the lower volume of water present now and thus lower volume of oxygen. As well, if the oxygen balance in the pond was reliant on the filters or any supplementary aeration, these being off could also play a role here.

In this situation, it would be ideal to check water temperature and water parameters (ammonia, nitrites, nitrates, pH, hardness) and it’d be even better if we had pre-change levels to compare it to (to see if there was active nitrogenous toxicity present that has led to a system shock). If anything is amiss, then it needs addressing. Since the ideal water temperature for koi is between 65 F-75F, you do want to make sure thew water is within this range. If it is not and you need to raise/lower it, do consider keeping to the lower end of ideal for them since they will have a lower body oxygen demand in the cooler temperatures (which may just give you a chance to get the oxygen angle sorted). Otherwise, you do want to take steps to make sure there is adequate oxygen in the water for these fish. This means turning the filters back on (as you have) and potentially adding further oxygen supplementation (ie add airstones, etc).

Overall, it sounds like these are fish in shock and potentially under the stress of compromised oxygen availability (likely related to the volume of fish struggling to cope with both the severe environmental change and the reduced water/oxygen access during the refill). Therefore, do check the above (or have the experts do so) and make the appropriate adjustments to restore the pond's default state. Hopefully, once everything is back online and the standard pond set-up restored your fish will be able to settle.

I hope this information is helpful.

If you need any additional information, do not hesitate to ask!

Dr. B.


Remember that if you have any lingering questions or concerns, please reply so that we may continue our conversation. I will be happy to work with your further and do everything I can to provide you with the service you seek. Please remember to rate my answer when you are satisfied (with 4-5 stars or a happy face) so that I may receive credit for my assistance. Thank you & have a great day. : )

Customer: replied 4 years ago.

Thank you for the information. Unfortunately, I found you too late -- all 32 of the fish have perished.


It sounds like the pond maintenance company should have been able to save the fish last night if they had taken water samples, etc. I am amazed that, I as a novice were able to keep the fish flourishing for 5 months and "experts" killed them in 24 hours!

Oh my goodness, I am so sorry to hear that they have all succumbed to this shock. I admit I was mortified when I read your initial question with what was transpiring for these fish. I agree that one would expect a pond maintenance organization to be aware of the delicate nature of fish and take proper preventative steps (ie making sure all water was treated and a proper temperature before adding it to the pond) when making changes to their pond. I do think this is something you will need to discuss with this organization since their approach has lead to so many mortalities.

Please take care,
Dr. B.
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