Activated carbon, also called activated charcoal, is a natural toxin remover. It is often kept in hospital and veterinarian emergency rooms. There is no downside if taken internally  – it can’t hurt – and is very effective in absorbing toxins. Often the mere indication that a person or animal has ingested toxins is reason enough for a doctor or vet to cause the patient to ingest activated charcoal.
cat fountain filtersIn a cat water fountain it removes impurities which cause unpleasant tastes and odors. The carbon we use in our cat fountain carbon filters is high density, pelleted, activated carbon contained in a reusable nylon sac. It actively draws and absorbs impurities. We use about 10 grams of carbon for each filter (20 per order of two) many times that used in commercial filters. This is why our cat fountain carbon filters last at least three months whereas many commercial cat fountain makers require replacing the filter every 3 weeks. 
Because the water is constantly circulating in the fountain, all the water is continuously coming into contact with the filter and being cleansed. (The method used in aquariums with this same filtration media is to simply hang the filter over the rim of the aquarium and let it dangle in the water.) Many of our customers use bottled or filtered water in their cat fountains so don’t require a carbon filter. This is why we don’t include them with every fountain, which would also require raising the price of the cat fountains.
A carbon filter does not prevent cat hair or debris from entering the pump. Our foam pre-filters, which are included with every fountain, serve that function. Many of our customers have left reviews about the effectiveness of our carbon and foam filters and they are all exceedingly positive. They work, and they work well. 
ThirstyCat Fountains