Is Copper Sulfate Toxic To Fish?
Copper sulfate has been used for many years as an algaecide and a parasite treatment. The problem with the use of copper is that there is a thin line that separates effective treatment levels from overdoses, which can kill fish.
Copper ions are highly toxic to fish, so care must be taken with the dosage.
The toxicity of copper to fish increases as the total alkalinity decreases. If the total alkalinity is less than 50 ppm, copper treatments are not recommended because of the high risk of killing fish.
In a pond with algae, copper treatments can cause oxygen concentrations to drop, which may result in fish kills. Pond algae is a major source of oxygen production and by removing it this source of oxygen is also removed. In addition, oxygen will be consumed as the algae decomposes.
Copper is also toxic to invertebrates, such as snails, and also most of the zooplankton in a pond (daphnia, rotifers, etc.). If you are relying on the zooplankton as a food source, then don't use copper.
In conclusion, caution must be exercised if using copper treatments due to its effects on fish and other life in the water. If your water is low in alkalinity, or if you have a heavy algae bloom and no aeration, copper treatments are not recommended.