A water diffuser that extends from the discharge outlet of a pet fountain, for receiving water from the outlet of the pet fountain's water pump, wherein the diffuser includes a perforate tubular diffuser section that can be releasably coupled or otherwise anchored to part of the fountain by a connecting or coupling section that preferably is tubular and imperforate. The diffuser can include a water treatment disposed within the diffuser that is dispensed or released into water flowing through the diffuser. The diffuser may also include a filter for removing contaminants from water flowing through the water. Where used together with a water treatment, the filter can be disposed within the diffuser upstream of the water treatment for filtering water flowing through the diffuser before treating the flowing water.