The Drinkwell® Premium Filters feature six compartments of charcoal! This evenly distributes the charcoal for a longer lasting, deeper water filtration. More charcoal is added to each compartment to maximize water filtration. This filter will work with ANY Drinkwell® Fountain (except the Drinkwell® 360) and should be changed every 2-4 weeks to maintain water freshness. There are three filters per package.