perfect for the fish but what about the plants? Some plants want it
closer to neutral, at least that's what I've read. What does plaster of
paris keep the ph at?

Nedra wrote:
I have to constantly add to the KH (which also impacts the pH)
to keep the pond buffered.

I add 1 cup of baking soda (the Arm & Hammer stuff)
per 1,000 gallons of water once a week. This keeps
the pH at 8.4 - perfect for the Koi and Goldfish.

Nedra