A method of eliminating, reducing or preventing parasites in a fish population is provided by feeding emamectin or a salt thereof to the fish population at a daily dose of 25 mu g to 400 mu g per kg of fish biomass per day for a period of 3-14 days. Also provided is a kit for preparing a medicated fish feed for eliminating, reducing or preventing parasites in a fish population, the kit having a supply of emamectin or a salt thereof and printed instructions for feeding the emamectin or emamectin salt at a daily dose of 25 mu g to 400 mu g per kg of fish biomass per day for a period of 3-14 days.