Abstract: Apparatus for mixing fresh water with CO.sub.2 gas to produce carbonated water in a cooled storage tank, includes a space located along the wall of the storage tank adjacent the cooling coils of a cooling system where water flow is partially or fully impeded so that ice can crystallize during the formation of an ice bank in the storage tank. In one configuration, the space is provided by a small section of open pipe mounted on the storage tank wall in the area of the cooling coils. Another configuration includes an apertured structure of rectangular cross section located on the tank wall. Still another configuration consists of an outward projection of a section of the wall of the storage tank which extends between two cooling coils, with an apertured plate covering this projection. Such a space can also be realized by a roughened wall section surface in the immediate vicinity of the cooling coils.