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I apologize that no one has responded to your question sooner. Different experts come online at various times. I just logged on and saw your question. My name is ***** ***** I’m a biologist with a special interest in fish. I'm sorry to hear of this problem. This means your koi are weak for some reason. We'll have to do some detective work to figure it out.
With any koi problem, the first step is to test the water. After you do that, let me know the numbers in parts per million (ppm) of ammonia, nitrites, and nitrates.
Do the fish look normal in every way - fins upright, no sores or lesions, eyes clear, etc.?
Thank you for getting back to me. If fish are dying, you really need to get an accurate test kit. Those strips are unreliable. You'll need to use a test kit, such as the one made by API. It uses test tubes and chemicals you add to water samples. It will give you results in ppm. If you can get those results, I'll better be able to help.
The only thing you can do in the meantime, is add aquarium salt at the rate of one teaspoon peer gallon of water. That helps fish cope with stress. If you can get accurate test results, let me know.
I'm glad to hear you have salt on hand. I'll watch for your test results tomorrow.
Oh, wow! I'm so glad you mentioned the ion-gen to them. Copper is one of the worst toxins that can affect a pond. Many koi keepers and authorities will never use ion-gens for that reason. Here is a site where you can read the scientific evidence concerning them:
Don't stop with one water change, though. The copper builds up in the fish's bodies over time. I would change 30% of the water again tomorrow, then 10% for several days afterward - as long as the koi show any symptoms at all. You can also use a product called Laguna Pond Detox. It helps beneficial bacteria break down harmful compounds in the water. It was originally designed for other toxins, but it can't hurt anything.
I hope you won't lose any more of your fish.