Tank for fish farming

A tank (1) for fish farming, wherein the tank (1) comprises: * a. a polygonal or circular ring pontoon (2) forming the lower part of the tank, * b. sidewalls (4) being substantially vertically arranged on the ring pontoon (2) to form a substantially vertically arranged tubular member having a circular or polygonal cross section, and * c. vertical columns (3) being arranged perpendicular upwards on the ring pontoon (2) and being connected to the sidewalls (4), and extends from the ring pontoon to the top of the sidewalls, where the ring pontoon and the vertical columns comprise closed buoyance elements, and where the ring pontoon is adopted to be ballasted by means of water being pumped into the buoyance element(s) therein, is described.