I would spray it with diquat. But, I would only spray about 20 to 25% at a time. Dieing vegetation robs O2 from the water. The fish need someplace to go to for oxygen. Wait about 10 days(guesstimate) between applications.
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