1. [Round Pet Filter] The diameter is about 4.95 inches. The diameter of the hollow circle is about 0.4 inches. 2.[Premium High-Quality Filter] Triple filtration system: Consists of a cotton layer, activated carbon and an ion exchange resin. Effective in filtering hair, heavy metals, keeping the water fresh and improving the taste of the water. 3.[Replacement Frequency] To prevent activated carbon dust from flowing into the water, soak the cat fountain filter in water for 10 minutes and then rinse thoroughly under running water before using it. 1 to 2 weeks in summer and 2 to 4 weeks in winter. If multiple cats use the fountain, more frequent replacements are required. 4.[Premium 6-Pack Pet Fountain Filter] 6-pack premium pet fountain filters, individually wrapped and sealed to maintain high quality. Sufficient supply for 1.5 to 6 months of use.