Abstract: The present invention provides a process and apparatus for mixing gaseous mixtures into sea-water pumped from an aquarium through the contact tube of a protein skimmer. The invention forms air bubbles of a specific uniform diameter so as to obtain an optimum skimming. The apparatus has a centrifugal pump closed about the periphery of the vanes at the suction side. A suction port at the axial part of the impeller forms water intake ports which surround an internal gas injection connection projecting into an axial water suction connection of a central disk fixed within the suction port of the impeller. The height of the water column over the suction point can, within limits, be taken into account by different gas and water suction cross-sections, adjustment of the gas injection connection relative to the water suction connection, and different sizes of the impeller.