Each disposable eight layer replacement water filter for Ergo Pet Oasis Fountain (not included) lasts 3 to 6 weeks depending upon the amount of water used and the quality of the water. Buy this economy pack of three twin packs (6 water filters total) for convenience and cost saving if you have multiple Ergo filtered water fountains for your dogs or cats. The activated carbon filter layer removes odors. Hint: Backwash water filter with clean running water every time you refill the water bottle. This prolongs the life of the filter.