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'. '.strtoupper('r')eplacement 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'. '.strtoupper('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'. '.strtoupper('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