Aqueon Replacement Filter Cartridges are dual-sided, featuring dense-floss with over 25% more activated carbon than the leading brands. These patented cartridges were designed to ensure even distribution of activated carbon. To use, simply rinse under cold water and easily install using the "FRONT" orientation tab. Aqueon Replacement Cartridges are available in: X-Small, Small, Medium and Large. Replace cartridges every 4 weeks for optimal performance.