A protein skimmer for use in purification of water by culture of fish or other water organisms in a reservoir, aquarium or (2) with a circulation system comprising one or more discharge tubes (4) for water from the reservoir (2) to a pump (5) for circulation of the water, and a return tube (3) for water directly or indirectly from the pump (5) to the reservoir (2) The return tube (3) comprises at least one generally horizontal, straight portion (90) having one or more upwardly directed side branches (9) which has an opening (91) leading to air, in which the generally horizontal straight portion (90) is arranged for rapid passage of the water past. the side branch (9) and to have the water level at or above the intersection of the side branch (9) with the straight portion (90), and in which the side branch (9) is directed rearward relative to the direction of the water flow in the straight portion (90), and forms a first angle (v) with between 0.degree. and about 90.degree. with the straight portion (90).