The Catit PIXI Cat Drinking Fountain Filter provides your cat with cleaner, better tasting water. The filters fit all Catit PIXI Cat Drinking Fountains. The filters clean your cat's drinking water in three ways. First, cotton mesh filters out debris. Second, active carbon removes odors and impurities. Third, ion exchange resin softens tap water. Replace the filter every 30 days for optimal results. Only use Catit certified filters with your PIXI Fountain.