The Drinkwell Replacement Filters keep water fresh and clean and are designed to fit most Drinkwell fountains. Each filter is constructed with two filtration pads. First, a duo density polyester pad that's designed to catch large particles such as hair and other debris. Second, a carbon impregnated pad removes bad tastes and odors, making the water fresher and more appealing to drink. In between the pads there is added granulated charcoal for extended filtration. Filters should be changed every 2 to 4 weeks.