Every fish tank filter needs filter media, which is placed inside it to change the quality of water passing through. There are three types of filter media, mechanical, biological and chemical. All three types are recommended to maintain the smooth running of your fish tank and all three can be incorporated into the same filter to make sure you have the best filtration possible.
API have a selection of filter media. Most can be used in either salt or fresh water aquariums and are specially designed to fit in API Filters.