Interpet Replacement Carbon Foam Filter Pads are a chemical filtration device. Chemical filtration takes place through the black foam which has been impregnated with carbon. Carbon foam filter pads help remove toxic heavy metals, odours, chlorine, unwanted treatments and water discolouration. Please remove your carbon filter when treating your aquarium as the carbon will remove the treatment from the water, rendering them ineffective. Please replace the Interpet Carbon Filter every 8 weeks in the back of your PF filter. 1 pack contains 3 foam pads.