h20fwlkillr :

ML is correct. Copper sulfate can cause fish to be unable to uptake O2 . It is not an O2 depletion , but that in the area of treatment when the fish's gills contact to much copper sulfate in the water they can't breath . That may be why some fish were not effected. Chelated copper is much less likely to cause a problem than copper sulfate. A second way it can cause a problem but not as quickly is by O2 depletion. When the plant material dies it stops producing O2 and starts using up O2 in decomposition . ewest