The 2 together can be a lethal killer. Also in low alkalinity water Cu products can inhibit fish gill function. Gill problems do not happen nearly as much with chelated copper products vs CuSO4.

One rule of thumb is to never treat more than 20% of a pond in one application. I try to stay around 10%. Also treat the problems early when the water is cooler as the water holds O2 better when a little cooler. I try to treat FA early when it is on the pond bottom because you are not as likely to kill the top 3 feet of the water (including surface plankton).