Closed tank (11) for fish farming, in which the tank is equipped with a liquid tight housing (12) and furthermore is provided with 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 (13) which serve as a load-bearing element of the tank (11). Preferably the inlet pipes (13) together with a vertical pipe (14) and interconnected pipes (15a, b) constitute a load-bearing skeleton of the tank. The tank is typically provided with separate buoyance chambers (48) dimensioned to provide sufficient buoyance even if one chamber should fail, and one chamber (48') in contact with the water in the tank arranged to be filled with and drained for air respectively, to thereby reduce and increase the water level in the tank respectively and the use thereof for transportation of fish out from and into the tank. A method for transportation of fish into and out from the tank is also contemplated.
