I think it COULD be from over fertilizing or maybe from fertilizing when it wasn't needed. The brown COULD be dead phyto and/or zoo plankton. Did it appear to be a nice green, then a darker green and then brown?
What was the visibility like before you fertilized?
Some of it looks like FA but FA needs nutrients like dead stuff provides. How was the FA before this happened?
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