An apparatus (1) for treating a fish infected with an external parasite is described, the ap-paratus (1) comprising: - an inlet channel (2) for the fish； - a draining unit (3) for draining a water； - a treatment chamber (4)； - at least one light source placed inside the treatment chamber (4) for irradiating the fish； and - an outlet (5), wherein the at least one light source is configured to irradiate the fish with an infrared light with a wavelength of between 0.7 and 3 micrometres. A method for using the apparatus (1) is described as well.
