Zanyzap's 6 Pack of Nitrate Remover Infused Pads for Fluval FX4, FX5, and FX6 Canister Filters are designed to trap floating particles such as debris, waste, and food, as well as reducing the amount of nitrites found in your tank. These are also helpful in preventing outbreaks or chemical spikes, and fish loss. Custom made to tightly fit the center white chamber, this filter pad will prevent unwanted bypass and enhance your overall filtration. For best results, thoroughly rinse pad before use and replace monthly or as required after testing water with a water test kit.