Copper is an old but effective treatment for fish diseases. It is now primarily used to treat acute infestations of parasites such as Amyloodinium and Cryptocaryon. Many fish are sensitive to copper and may become intoxicated at concentrations required to control pathogens. Copper concentrations in treatment tanks must be measured frequently to ensure that levels are therapeutic but not toxic. A knowledge of the solution chemistry of copper is essential in understanding how to avoid unexpected changes in copper concentrations. Factors such as pH, salinity, decorative materials, nature of the filter bed, and the type of therapeutic agent used for treatment must be considered.