Copper Sulfate ( CuSO4 ) can cause problems in a number of ways.

One is quick death for fish and lasts a day or so. It happens most often with low alkalinity and CuSO4 combined. It causes the fish's gills to be unable to uptake oxygen and thus a quick death like a DO kill.

A second problem is longer in the making but can kill the entire pond. This occurs when a large area of the pond is treated for weeds , the weeds die as well as the plankton , they not only quit making o2 but as dead plant material uses up the o2 in the decomposition phase you get a full blown DO crash.

Yet another problem is the build up of copper from multiple uses in the pond bottom soil which can be toxic as well.

Still another problem is the effect of treating with CuSO4 when the fish are spawning with eggs/ small fry /yoy as they are much more susceptible to its effects.

GC are less tolerant of CuSO4 than some other fish.

Chelated copper products are much better. If you use CuSO4 learn about its effects first.

See this fact sheet from SRAC:

http://srac.tamu.edu/tmppdfs/11830457-361fs.pdf

See these with emphasis on Bill Cody and Kelly Duffie's posts :
:
http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=20;t=001155#000001

http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=20;t=002023#000006

http://www.pondboss.com/ubb/ultimatebb.php?ubb=get_topic;f=20;t=002572#000009