A fluid-agitating tank assembly is disclosed. The tank assembly includes a tank filled with a fluid, and an agitator disposed in the tank so as to be at least partially immersed in the fluid and supported by the tank in a rotatable manner about an axis. The agitator includes a double helix disposed in an upper volume of the tank and adapted to act on the fluid to transform unidirectional rotation of the agitator about the axis into a bidirectional movement of the fluid in the tank towards bottom and top portions of the tank, and at least one rotary blade projecting transversally from the axis, disposed in a lower volume of the tank, and rotating about the axis to move the fluid towards at least one lateral outlet of the tank.