The Eheim undergravel filter forces a slight current of water to rise through the bottom gravel from below. As a result oxygen-requiring bacteria, which process all organic waste products to biologically harmless substances, settle there. The bottom gravel will no longer become clogged, and as well as acting as an extra biological filter, it aslo encourages rich plant growth. The throughflow principe is suitable for all fresh- and sea water aquaria, particulary for planted aquaria. The undergravel filter is not intented to be used when keeping fish which stir up the water a great deal. Owing to ist high effectiveness, the undergravel filter should be turned off when medication is used. The basic set includes 24 plates 9x9 cm each. This is sufficient for an aquarium with 80x35 cm. The extention set 3545 is to be used for larger aquaria.