Juwel filter sponges are made to fit each filter system. Use the filter baskets for your filter system to make maintenance even easier. The coarse and the fine blue filter sponges house the most important bio - cultures, which are responsible to effectively clean the water. You should clean or replace one sponge at a time every 3 - 6 months to maintain the right biological balance in your aquarium. Suitable for Rio 300, Rio 400, Delta 300, Trigon 350. 1 piece in each box.