An arrangement for delousing of caged fish by cooling the processing water (salt or fresh) to below sea temperature in a closed system. The lice get a cold-water shock and let go. The arrangement comprises an inlet 1 for fish to be treated, a first water separator 3 which separates the water from the fish and which is arranged behind the inlet, a processing pipe 4 that the fish passes through, the processing pipe containing processing water for the treatment of the fish, a second water separator 5 arranged at the outlet of the processing pipe and which separates the processing water from the fish, an outlet 11 for treated fish arranged behind the second water separator, a refrigeration unit 9 for cooling the processing water from the second water separator, and a circulation pump 8 which pumps the processing water through the refrigeration unit back to the inlet of the processing pipe. The arrangement may further include first 6, 7 and second filters 13 for filtering the water that passes though the separators, and an oxygen supply unit 14. The processing pipe may be zigzag shaped.
