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The presence of bubbles represents the suction draw through the plumbing that is attached to the ozonator. We are either drawing ozone through the operating ozonator or we are drawing ambient air through an non-operating ozone and the non-working ozone is "out to lunch" injecting air into the tub which does nothing.To confirm an operating ozonator check the ozonator (CD) for bussing sound and (UV) for the glow of the light