A fish rearing tank comprising the features of: a) an egg-shaped shell (1) with a generally vertical long axis and gradually narrowing shape towards its tip volume portion; b) said shell (1) forming a generally rigid tank; c) said shell (1) being closed; d) said shell (1) having one or more water inlets (11); e) said shell (1) having one or more water outlets (16, 29); said egg-shaped tank (1) for holding a water volume in its major lower volume portion and enclosing air in its minor, upper tip volume portion (4). Further disclosed is a method of rearing fish providing such an egg-shaped shell (1) and conducting bottom to top or ?reverse?-circulation of water through said egg-shaped shell (1) while maintaining its air-filled volume in said tip (4).