Juwel Nitrax XL Compact Nitrate Sponge for Bioflow XL (8.0). The sponge is designed to reduce nitrate levels in the aquarium. This in turn helps reduce the amount of algae and helps provide a more healthy aquarium for your fish and plants. Nitrax is also a biological filter containing specialist microorganisms to break down poisonous metabolites in the aquarium. It’s recommended to change the Nitrax sponge every 2 months for optimum performance. Suitable for JUWEL Rio 400/450, Vision 450 & Trigon 350.