Michael Shaffer wrote:

The first frost killed all my water hyacinth, should I leave them or
could the rotting be bad for my Koi?

I don't think it will harm the koi, but it will polute the water, burden
the filter, and could trigger an algae bloom.

To leave it in would be like throwing in a bucket of compost.
--
Pardon my spam deterrent; send email to
18,000 gallon (17'x 47'x 2-4') lily pond garden in Zone 6
Cheers, Steve Henning in Reading, PA USA