Which water chemistry condition is most associated with fish kills when using copper algicides?

• A. Carbonate content above 100 ppm.
• B. Carbonate content less than 50 ppm.
• C. pH above 9.0
• D. Very hard water.

Answer: (B)

Copper toxicity to fish is driven by how water chemistry affects copper’s form and availability. When carbonate alkalinity is low, the water has limited capacity to bind copper and form less toxic complexes. This leaves more copper in the free ionic form (Cu2+), which is highly toxic to fish and can lead to kills during copper-based algicide treatments. In contrast, higher carbonate levels help form complexes that reduce the amount of free, toxic copper, lowering risk. So the condition most associated with fish kills is low carbonate content, because the water can’t buffer or complex the copper effectively. If alkalinity is below about 50 ppm as CaCO3, exercise extra caution, test before application, and consider adjusting alkalinity or using alternative control methods to protect aquatic life.