All Catit filters should be replaced every three weeks or as soon as there are signs of other foreign substances on it (dirt, bits of old food, hairs, etc); Filters may need to be changed on a more regular basis depending on the environment, water quality or if it is serving several animals. Replacement Filter A: Intended for use with the Catit Design Large Dog Drinking Fountain 10.5l and the Catit Design Drinking Fountain with Food Bowl 3l. 3-Pack Dual-function filter: Foam captures dirt, food particles and debris Activated carbon helps reduce bad odours, tastes and absorbs other tap water impurities Replacement Filter B: Intended for use with the Catit Design Fresh & Clear Drinking Fountain 3l and the Catit Design Drinking Fountain for Cats & Small Dogs 2l. 3-Pack Dual-function filter: Foam captures dirt, food particles and debris Activated carbon helps reduce bad odours, tastes and absorbs other tap water impurities