Device for destroying parasites on fish, such as salmon lice on salmon in fish farms, comprising a camera (4) communicating with a controlling unit (51) which in turn communicates with a light source (5) adapted to fire pulses of point shaped light which is harmful for the parasite in question. The controlling unit (51) controls a system for optical recognition within a defined coordinate system and is arranged to detect points and to update in real time coordinates that exhibits contrast differences typical for parasites on a fish surface and to trigger a light pulse from the light source (5) when the coordinates for at detected point coincides with the coordinates for the aiming point of the light source.