| I really hate this for you, as you have obviously worked hard to prevent this, but the more we read the more it begins to look like a "perfect storm" situation.
A heavily stocked pond with ph on the low side of comfortable.
Kh value likewise on the low end of the scale.
A very low ph fresh water supply.
Recent spawning activity causing added bioload to the filter.
Filter washed from a garden hose either killing off much of the bio from chlorine (if municipal water) or from acidic wash water (likely if from a well laden with acidic CO2)
Add a nice bit of acidic rain to the mix and multiple crashes in kh, ph, bio were almost inevitable.
A few suggestions.
Test your tap water again after letting it rest for a few hours for both kh and ph. I'm betting the 6.0 ph will rise to around the 6.8 you have in the pond.
In the future pay closer attention to Ammonia and Nitrite, and less to Nitrate. The first two pose an immediate health risk to Koi, while Nitrate is far less an issue.
Since your water runs on the acidic side monitor your kh religiously, and try to maintain it above 125.
In the future, rinse your filters with pond water rather than from the hose. That way the bacteria will not be exposed to either chlorine or highly acidic conditions.
I hope all of this helps you to have a better future with Koi even though the lesson has been painful.
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Larry Iles
Oklahoma
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