Many people who say the use of copper sulfate will kill your fish have probably experienced a fish kill shortly after using an algaecide containing copper sulfate. 99% of the time the fish kill is caused by user error not the actual product. While the application of copper sulfate can be toxic to fish you can take some very simple precautions to ensure your fish are safe and healthy.
Copper sulfate is toxic to very few fish. Some species that you can’t use copper with are Koi, goldfish and trout all of which are listed on the label. Pond owners that have experienced a fish kill after using a copper sulfate algaecide most likely treated too large of an area. When a pond is covered in algae it is important to only treat a portion of the pond at a time. If too much of the pond is treated at one time the decaying algae will deplete the water of oxygen.
The symptoms of a pond that has oxygen depletion will be indicated by the fish. Fish will stop eating and will come to the surface and look like they are sucking air. The reason they come to the surface is because the surface water is the most oxygenated. You will also notice larger fish will die first. Larger fish require more oxygen and are therefore affected much more quickly. The only way to reverse oxygen depletion is by installing an aerator.
For more information on the use of copper sulfate Contact Sanco.