Copper and zinc generated by the Aquascape IonGen pond clarifier system can be detrimental to koi (Cyprinus carpio) health.

Background: Copper is frequently used as an algicide, and copper ion generators such as the Aquascape IonGen claim to be safe for use in systems containing fish. In 2012, a die-off of koi (Cyprinus carpio) in a pond in Raleigh, North Carolina, occurred after the IonGen was added to the system.

Methods: Physical and postmortem examinations suggested that heavy metal toxicity was the likely cause of morbidity and mortality. This was supported by a heavy metal screening of the owners' pond. Additional experiments were performed to determine if the IonGen produced toxic levels of copper and zinc.

Results: The tank containing the IonGen had higher concentrations of copper and zinc, and copper levels exceeded those associated with toxicity in both hard and soft water.

Conclusion: The results of this study indicate that ion generators might not be safe for fish, and copper should only be used as an algicide if concentrations are closely monitored.