Juwel filter sponges are made to fit each filter system. Use the filter baskets for your filter system to make maintenance even easier. The active carbon sponge cleans the water of all chemical impurities by absorbing them in its active carbon. Consequently, these chemcial impurities will not effect the bacteria - cultures inside the blue sponges. If the carbon sponge is left in the filter too long, it can release the chemicals again into the water. We recommend to replace the carbon sponge every 4 weeks. Suitable for Rio 300, Rio 400, Delta 300, Trigon 350. 2 pieces in each box.