Use these replacement filter cartridges in your Tetra water filter each package is color-coded to make it simple to remember which size to purchase. These replacement filters are assembled and ready to use for your convenience. Clear water made simple. Rinse Tetra Whisper Bio-Bag Disposable Filter Cartridges under cool tap water to remove carbon dust. Open the lid of your filter and insert the Bio-Bag cartridge into the filter. For best results, change your filter cartridge monthly or sooner if needed. Size medium. Choose the small size filter cartridge to fit the Tetra Whisper 3i Internal Filter; choose the medium cartridge to fit the Tetra Whisper PF10 Power Filter, 10i Internal Filter, and Reptofilter 90 GPH; and choose the large cartridge to fit the Tetra Whisper 20, 30, 40 and 60 Power Filters and 20i and 40i Internal Filters. Since 1951, Tetra has developed the world's most comprehensive body of fish food knowledge, and fishkeeping enthusiasts have looked to the brand for products and solutions that add ease and beauty to their homes.