I think the washed in fertilizer would explain your bumper crop of duckweed and algae. However, I don't think that was a factor in the fish kill?

Your gasping fish seems to confirm the low dissolved oxygen problem.

Hopefully, some of the experts can drop in and give some advice on proper procedures after a similar future event to help you and others avoid a fish kill.

Sorry about the loss of your big fish. Hopefully you can post an update in the future reporting that you still have decent numbers of your small fish remaining!