Algae infestations are a common problem in catfish culture ponds. To treat filamentous algae, frequent applications of copper sulfate are necessary. Free copper is toxic to fish, and its concentration is dependent on a variety of factors such as its reaction with carbonate ions, absorption by phytoplankton, and temperature. The Frankfort Fish Hatchery has been using a colorimeter to measure the concentration of free copper ion after the application of copper sulfate to ponds stocked with blue catfish and channel catfish fingerlings. The use of the colorimeter allows managers to determine when the free copper concentration has decreased enough to safely treat the pond again. It has been observed at this hatchery that blue catfish are more sensitive to copper sulfate treatment than channel catfish. During the 2000 production season, the average survival of 3 blue catfish ponds treated with copper sulfate pentahydrate was 75%.