An apparatus and method for ozone-aerating fluid is connected to a conventional water treatment system that includes a high pressure fluid pump and a high pressure filter connected to a main reservoir. Included is an auxiliary reservoir and low pressure filter. A first valve controls the fluid flow in a conduit connected to the output of the high pressure filter. A second valve controls the flow of fluid output from the filter to a lift tube. Ozonated air bubbles are injected into fluid in the lift tube. The fluid flows up the lift tube and into the auxiliary reservoir due to expansion of air bubbles. The fluid flows out of the auxiliary reservoir and filter under the force of gravity into the conduit on the downstream side of the first valve and into the main reservoir. A UV germicidal lamp may be positioned in the path of fluid flow.