Couple of thoughts but I really don't know. I generally look for "What changed?" and the water seems to be the culprit in this case.

Have you checked the pond and well water for alkilinity and Ph? I have no idea what tolerance level plants can handle but a colder water change could possibly be a culprit. It might be safe to drink but have an adverse reaction for vegetation?

It MIGHT be possible that you did lose some fish since 95% of dead fish either sink or get eaten as they weaken.

It's not about the fish. It's about the pond. Take care of the pond and the fish will be fine. PB subscriber since before it was in color.

Without a sense of urgency, Nothing ever gets done.

Boy, if I say "sic em", you'd better look for something to bite. Sam Shelley Rancher and Farmer Muleshoe Texas 1892-1985 RIP