A separator tank (1) for separating oil and gas from water comprises an essentially cylindrical vertical tank with at least one separator tank unit. The separator tank unit has an inlet for fluid (2) and a first inner annular wall (5) a first conical portion (9) and a first central opening (8). A second inner annular wall (15) is positioned at a distance from the first inner annular wall and has a second conical portion (19) and a second central opening (18). A first flow opening (10) is provided at an end of said first inner annular wall (5) and a second flow opening (20) is provided at an end of second inner annular wall (15).