The ' Kaldnes Moving Bed' process has been tried & tested in commercial applications for over 10 years. Now it is available to the fish keeper. The media are wheel shaped & float letting them move around in the filter. This process constantly removes old dead bacteria from the media & makes room for young healthy bacteria that will grow & feed rapidly on the harmful ammonia & nitrite introduced by fish waste. Unlike foam matting of other forms of static filtration media the Kaldnes K1/K3 media moves freely within your filter. The constant chaotic movement of the air from an air pump causes the media to self clean so it requires no maintenace. For adding to an existing filter K3 is the most suitable due to its large size & ease of control (K3 is the larger of the two in the product image). Available in 50 litre bags