3-Pcs Charcoal Water Filter Replacement Pet Fountain Filters by APPLIANCEMATES are sealed with charcoal inside. They have a frame and are custom to fit exactly in the slot for a certain pet fountain. <br><br><b>REPLACEMENT FREQUENCY</b><br><br> ► It depends on how many cats and dogs are using it, and how big your pets are, and so on. <br><br>► Change the water in the fountain weekly and check the filter then, and change it when it looks dirty on time. Changing the fountain filters every 2-4 weeks is better.<br><br><b>Thanks for choosing this part for your needs. Any questions, please contact with us, we will reponse in 24 hours.</b>