Drinkwell Replacement Activated Carbon Filters keep your pet's water tasting clean and fresh. 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 extend the life of your fountain and pump and should be replaced every 2 to 4 weeks. To replace the filter, put the filter in the filter housing with the black side facing the front. Compatible with Drinkwell Original, Drinkwell Platinum®, Big Dog, Multi-Tier, Outdoor Dog, Mini, Everflow, and Zen fountains.