I'm Dr. Jo and I'm here to discuss your question about your fish.
I'm so sorry you're having trouble, but glad you're looking for the information you need.
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Could you please tell me how large this pond is?
Also, have you already had a water sample tested at a local aquarists?
While I await your reply, I'll offer the following: as I'm sure you already know, the sawdust from treated wood can be very deadly, and it doesn't take much exposure to kill fish. It can also last in the water for a long time.
In the US, fish pond water samples can also be sent to the local University Extension for analysis. They will provide a very thorough report that can help aquaculturists uncover the cause behind mortality problems.
This link will take you to an example of the kind of analysis done by our local Extension Office:
Here is a link to a home pond water analysis kit that can also be helpful. It isn't as exhaustive as the University report, but it still can be useful.
I should back up a moment and explain.
Your question was, "Any ideas?"
Unfortunately, there are numerous things that might be causing your fish mortality problem right now.
Too many to list.
But identifying the problem will require either...
1. Pond water analysis
2. Post mortem analysis of a deceased fish.
Otherwise you're only guessing.
For example, you tried medicating the water, but that doesn't appear to be solving the problem. It was a good guess and a good place to start, but now you clearly need more conclusive answers.
It has been said, "We don't keep fish. We keep water."
If your pond is small enough to drain and refill, that would solve the problem if this really is from exposure to the arsenic in the wood.
Unfortunately, if whatever is in the water is an ongoing exposure, it won't.
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I also think it's important to clarify that this type of fish mortality does not "sound like" any particular problem. We don't have any luck narrowing down most types of fish mortality problems by looking at the history and numbers of fish affected because they are not distinct.
Therefore, the best idea I have for you is to have your pond water analyzed. The most complete analysis that's available would be best.
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Are you there?
I answered al, your Q's but my reply disappeared from the screen. Did you get them?
Hope you got my answers second time around. I will be delighted to sign the funds off when I hear back from you. Thanks