Abstract: According to the invention there is proposed an arrangement in tanks for growing living fish. Each tank (20) is per se of conventional embodiment, with a peripheral inlet and a central outlet for water. According to the invention, one or more spiral grooves (26) are arranged along the bottom of the tank, said grooves extending from a point close to the periphery of the tank and in a spiral path towards the center of the tank to a sludge funnel (18) situated there. Water circulates in the tank (20), and a sludge sinking into the groove (26) or grooves is urged by the circulating water towards the funnel more quickly than if it had accompanied the water in its movement towards the outlet. The spiral groove (26) or grooves may be formed directly in the bottom of the tank (20), thereby causing great stiffness in it, or they may be formed in a separate means, which is placed on the bottom of the tank.