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).
