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Dr. B.
Dr. B., Board Certified Veterinarian
Category: Vet
Satisfied Customers: 21664
Experience:  General practice veterinary surgeon with extensive experience in a wide range of species.
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We have a pond 6' x 4' x3' deep. We had 40plus fish. Every

Customer Question

We have a pond 6' x 4' x3' deep. We had 40plus fish. Every 2-3 weeks a fish dies we are now down to 20. We have cleaned filter, tested water which was fine. Have you any ideas please what could be wrong. At the moment we have cleaned the pond out, refilled from the tap and leaving water over night.
Submitted: 5 months ago.
Category: Vet
Expert:  Dr. B. replied 5 months ago.

Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today. I do apologize that your question was not answered before. Different experts come online at various times; I just came online, read about your situation, and wanted to help.

Again I do apologize that my colleagues could not aid you sooner, but can you tell me:

What were the pH, ammonia, nitrites, and nitrate readings specifically?

Are the fish showing any lesions or changes prior to death?

What volume pond is it?

What type of filters and oxygen sources do you have?

Are you treating the tap water?

Customer: replied 5 months ago.
I do not have the test kit with me, but all the readings were within the normal range. There is nothing wrong with the fish when they are removed from pond dead.
The volume must 6x 4 = 24 x 3 = 72sq ft,I believe is approx 2300 litres.
The filter is a proper pond filter for that size pond. I have replaced the sponge filters inside. The oxygen comes from pond weed, which there was a lot of.
Last night the pond was cleaned, scrubbed, rinsed with tap water and refilled and is sitting for 24hrs to let any chlorine settle.
It is a difficult one as I cannot understand why they are going on a regular basis and not all together.
Expert:  Dr. B. replied 5 months ago.

Hello again,

That is fine, though I’d advise checking that your kit did check all those parameters as many don’t and thus a problem can be missed. As well, make sure to treat the water with dechlorinator before adding the fish back.

Anyway in this case, husbandry is key here if we are losing multiple fish with no signs of external disease nor changes before death. And I have to say that with those aside, my top concern for this pond is that you may have a inadequate oxygen level in the pond for the volume of fish present. This is a common issue for outside ponds with high stocking densities and goldfish rarely will air gulp at the top when this is a problem. Furthermore, it is a very common problem this time of year due to the warm water temperatures upping fish oxygen metabolism (compared to winter). And if you have a lot of pond weed, it may actually be our culprit here since they put oxygen into the water in daylight but take oxygen from the water overnight due to a lack of overnight photosynthesis. So, we may need to think about clearing some of this but most importantly, we’d want to add an oxygen support (ie air stones, water fountain, aerator, etc) at this stage to support the fish.

Kind regards,

Dr. B.

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Customer: replied 5 months ago.
OK i will half the weed and get an aerator and hope that does the trick. thank you.
Expert:  Dr. B. replied 5 months ago.
Hi again,
Sounds like a good plan of action. :)
All the best,
Dr. B.
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I aim to provide best care for you just as I do for my own clients & their pets. If you have any other questions, please ask – I’m happy to help but please rate me with the **STARS** (or leave a written rating of 1-5; 5 being great) as this is the only way I am credited for my work by the site & this allows me to continue to help here. This is included in your question fee. Once you rate, your question will not close & you can still ask follow-up questions. Thank you