"Michael Shaffer" wrote in message
The first frost killed all my water hyacinth, should I leave them or could
the rotting be bad for my Koi?

For anything to rot it requires oxygen, therefore rotting vegetation in
water is getting its oxygen from??? and koi get their oxygen from??? Bad
water is bad for koi, get the rotting WH, an anything else, out of your koi
pond. ~ jan

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(Do you know where your water quality is?)