A mixing apparatus for mixing first salinity fluid with a second salinity fluid, the mixing apparatus comprises a down tube having a side wall, an open upper end and an open lower end, and a plurality of apertures selectively located in the side wall of the down tube. At least some of the apertures have an arm with an opening therein about the aperture to facilitate inflow of fluid to the down tube through the openings. A fluid inlet and a fluid outlet are provided at the open upper end and open lower end respectively of the down tube so that the first salinity fluid in use enters the down tube through the fluid inlet and the apertures and is discharged therefrom through the fluid outlet. A feed tube is provided and has a first end connectable to a source of second salinity fluid having a salinity different to the first salinity fluid and a second end for introducing the second salinity fluid to the fluid inlet of the down tube to mix the first salinity fluid with the second salinity fluid to form a fluid mixture.