The filter sponges in red/purple for the BioTec ScreenMatic² 40000 flow-through filter are easy to use and very easy to clean. They are components of the fine filtration in the BioTec filter: Mechanical fine filtration and biological filtration through aerobic (oxygen-rich) conditions allow bacteria in the red zone to break down or oxidise to nitrate. Nitrate can be naturally absorbed and broken down by plants. Also suitable for the predecessor model BioTec ScreenMatic 12. Contents: 2 filter sponges