Japanese mats are an excellent filter medium. Their particularly dense fiber structure ensures good water flow and a large surface area for bacteria to settle. The mats do not need to be removed for cleaning as long as coarse dirt is captured by filter brushes. Therefore, cleaning is not necessary throughout the entire season. The mats are positioned to allow water to flow through easily, preventing the filter from clogging. Otherwise, a significant portion of the beneficial filter bacteria would be destroyed during each cleaning.