Petmate® Fresh Flow Replacement Filters use charcoal to keep your pets water fresh & clean. The charcoal filter removes odors to keep water fresh & provide naturally cool & oxygenated water. Encourages your pet to drink more water & stay hydrated while helping to extend the live of your fountain. Works with all Fresh Flow® fountains. Filters should be replaced every 30 days. Charcoal filter keeps water fresh & clean Fits all Fresh Flow Fountains Filter provides naturally cool & oxygenated water Change filter every 30 days to ensure freshness.