Apparatus for the treatment of parasites on fish comprises a conduit 100 having first and second open ends and an internal passage for accommodating a fish 101, a light source 103 adapted to illuminate a section of the internal passage radially inwardly from substantially all sides simultaneously, and providing light of wavelength lethal to parasites but harmless to fish and a detecting means 102 for detecting the fish The detecting means is operationally associated with the light source by a control means and is configured such that the light source is activated when the eyes of the fish are not within the region of exposure to the light source. The light source is static so as to avoid the need for moving parts, and covers a substantial portion of the conduit so as to provide indiscriminate exposure to a substantial portion of the body of the fish, avoiding the need for a means to detect the parasites on the body of the fish. The controls means if configured to activate the light source when a fish is detected by the detecting means. Means to count the fish and image capture means may also be provided.
