Aquarium Water Filter 4 Stage Replacement Set This is the 10" filter replacement set for the Aquarium Water Filter 4 Stage Unit. First Step: PPF Filter The PPF filter removes large particles like sediment, rust, and more, protecting the other filters. Second Step: Dechlorination Ball This filter quickly removes any leftover chlorine in the water. Third Step: CTO Filter Made from high-quality coconut shell activated carbon, this filter removes chlorine, bleach, organic matter, and some heavy metals from the water. Fourth Step: Soft Water Resin This resin uses ion exchange to soften water by removing calcium and magnesium ions.