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I've had a small pond set up in current location for about 9 months. I transferred my koi from another pond. Some of them are 10-12 years old, others are only 5-6 years old.

4 weeks ago, two of my koi (one older and one younger) died suddenly. Last week, another older one died. Yesterday, a younger one died and today one older and one younger will be dead within the next 2 hours or so. I have not sprayed anything on the lawn. I changed 50% of the water after the first two died. I use well water, so chlorine not the issue.

Weather was in low 80's °F (around 27 °C) when first 2 died. Now in mid 50's °F (around 12 °C).

I filter and have a small waterfall running constantly.

Anyone have any ideas why my fish are dying off?

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    we need more information to solve this,result of your water test nitrite-ammonia-ph.hardness of your well water and mineral content of your water.have you stopped feeding your koi now that the temparature is at/below 10celsius in the water,how big is your pond and what type of filtration do you use have you cleaned the filter lately.any changes in the behaviour of your koi(going to the surface of the water or staying still on the bottom.
    – trond hansen
    Oct 23, 2019 at 5:07
  • Hoping I found the problem although I dont know what caused it. Ph was so low it was off my test chart. Ph of my water coming out of the hose is 7.0 so, as I said, i dont know what would cause the ph to go so low. There is no bottom sludge. I use leaf netting so there is minimal amount of leaves in pond. To answer your questions, pond is only about 700 gallons, I use a pressurized bio filter, started with 9 fish, now down to 2. I have not cleaned the filter lately. Fish dont seem to be showing any change in behavior. Stopped feeding 3 weeks ago.
    – Grampaschu
    Oct 26, 2019 at 0:06
  • You might have moved to a soft water area. There’s not enough "buffer" in the water. I had the same problem with a fish tank in a soft water area. Fixed it with coral gravel (which acts as a buffer but stops dissolving at higher pH). Not sure what you do for a pond.
    – Pam
    Oct 26, 2019 at 11:35
  • I dont know; we soften the water we use in the house because of the iron in the well water. City water is not available where we are so that's not an option.
    – Grampaschu
    Oct 26, 2019 at 20:55

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You don't show the size of pond, number and size of koi, or amount of pond circulation. A "small" waterfall is good, but I would add an aeration pump, you can never have too much oxygen. If the well water is used by people with no treatment it is likely good. However, well water can have pretty nasty stuff in it. Another possibility is bleach or soap; I never use household buckets, etc. for the pond, I have even heard of people bleaching wood and decoration to be used in a pond. My pond temperature goes from the 80's to 50 F with no problem, so it is not temperature.

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