A mixing apparatus comprising a central disc driven by a rotating shaft. A number of mixing plates are stacked above and/or below the central disc. The mixing plates are spaced from and parallel to the central disc. Each plate has a central aperture and the diameter of the central aperture increases progressively away from the central disc to define a space coaxial with the rotating shaft. In operation, fluid is drawn into the space and directed out through the space between the plates. When placed near the surface of a fluid, air is drawn in and the fluid is aerated as well as mixed.