The filter is a key part of your pet drinking fountain, keeping the water in perfect purified order. At Closer Pets, they recommend replacing your filter every month, so this handy six-pack will give you half a year of clean and free-flowing water for your four-legged friends to enjoy. These replacement filter cartridges are suitable for soft water, but if you have hard water in your area, our 3-Stage Filter Cartridges (ref 386) will be perfect for you! For use with the below products if you live in a soft-water area: Three-level Two-litre Pet Fountain (335) Large Two-level Six-litre Pet Fountain (Ref. 385) Two-level Three-litre Shell Pet Fountain – White (410) Two-level Three-litre Shell Pet Fountain – Blue Bowl (412).