The PetSafe® Drinkwell® Replacement Filter Pack includes 1 foam pre-filter and 3 carbon filters for the PetSafe® Drinkwell® Outdoor Dog and Overflow Pet Fountains. Each filter features a duo-density polyester pad that catches hair and debris, while the carbon removes bad tastes and odors from the water making it more appealing to drink. Filters help keep your pet's water clean and fresh and also extend the life of your fountain. Replace them every 2-4 weeks. Trust PetSafe® to help keep your pet healthy, safe and happy.