The present invention relates to a composition comprising one or more capsaicinoids, or analogues or salts thereof, and acetic acid for use in prevention and treatment of infestations or infections of parasites on fish. Treatment of salmon lice, Lepeophtherius salmonis, in water baths comprising said composition is disclosed. The claimed invention also relates to methods for preventing parasites from attacking and attaching to the fish and/or methods for loosening parasites from the fish as well as methods for killing parasites.