Closed tank for fish farming, in which the tank is equipped with a liquid tight housing and a water supply system, a water discharge system and at least a particle discharge means, the water supply system comprising a plurality of vertical inlet pipes which serve as a load-bearing element of the tank. The inlet pipes and a vertical pipe and interconnected pipes may constitute a load-bearing skeleton. The tank may have separate buoyance chambers dimensioned to provide sufficient buoyance even if one chamber should fail, and one chamber in contact with the water in the tank arranged to be filled with and drained of air, to thereby reduce and increase the water level in the tank.
