Our fountain filter fits most 2-2.5 liter cat fountains! Dimensions: outer diameter 125mm, inner diameter 33mm, Better Activated Carbon Filtration + Ion Exchange Resin. Expel debris, absorb heavy metals, eliminate odors. Protect your little cat from unnecessary infections. Give them healthier, tastier water juice! Rinsing and Facing Up Rinse the fountain filter for 10 minutes before use to remove charcoal dust. Please put the flat face up to ensure the best filtration! 2-4 week change Although our fountain filter can last a long time, to ensure the best protective filtration for our pets, we suggest a change every 2-4 weeks. With individual packaging, each fountain filter can be easily stored and well reserved with the best effect!