Pura's nifty AddOn-3 is a three-gallon-per-minute system, each designed as a stand-alone UV unit or as a convenient addition to an existing drinking water unit. These are compact, easily installed systems that are trouble free and easy to service. They have no filters to change and require only an annual lamp change. The Add-On units are fitted with John Guest type 3/8" push-in quick connect fittings. Simply push your 3/8" plastic or polypropylene tubing into the fitting for a tight, leak-free seal.