Even the best internal and external filters are ineffective, if they are not filled with filter media. These are made from various materials which perform various functions. They trap dirt and fine particles, provide biological decomposition of harmful substances, remove other contaminants or effect a chemical change. Depending on the size and stocking of the aquarium, the type of filter as well as the local water quality, appropriate filter media need to be used and combined. The correct layering of the filter media is critical for the water quality.