If you already own or are purchasing a Catit Fresh & Clear Drinking Water Fountain for Cats, Small, sold separately, it is important to change your filters about every three weeks.  The Replacement Filters for Catit Fresh & Clear Drinking Water Fountain for Cats are foam/carbon based filters that help reduce bad tastes, odors and absorbing impurities present in tap water.  There are three filters in the package, which should last you a little over two months. Using these filters with your fountain will provide clean and fresh water that is essential to preventing urinary diseases and maintaining a healthy lifestyle for your pet. 
Please Note: More frequent changes may be necessary due to indoor environmental and water conditions, as well as multiple pet use.