These carbon filters are for ThirstyCat fountains only.
Activated carbon, also called activated charcoal is a natural detoxicant. It is a staple in vet and medical emergency rooms as it can be ingested without risk and will absorb toxins, poisons and impurities. In cat fountains their purpose is to absorb impurities which can cause unpleasant odors and tastes.
These cat fountain carbon filters are optional and only necessary if you have impurities in your water. If you do have impurities, or if impurities tend to generate from usage, they help eliminate them and the undesirable odors and tastes they cause. These filters are comprised of a reusable nylon mesh sack filled with high density pelleted, activated carbon containing no phosphates. (See this link to understand how they work.)
This is premium grade carbon which lasts longer and performs better than other types and which actually attracts impurities and absorbs them. As the filter sits beside the pump the entire volume of water is constantly coming into contact with the filter, which attracts and eliminates the impurities. This listing is for 2 filters of approximately 10 grams each (20 grams for the order of two). Each filter should last about 3 months.
This is the same carbon used in high-end aquariums where it is suspended in the water in a nylon mesh sac.
The second pictures hows how it fits over the foam filter. A foam filter comes with every fountain except the Travel Fountains.