With a pond of 1000 liters, I would just replace ALL the water. With my 36 000 liter/hour borehole, it will take me 2 minutes to fill again. But, the filter will have to be cleaned properly, and that will take a while.
Who keeps koi in a 1000 liter pond?
If it is 10 000 liters, then Wayne's suggestion makes sense.
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