The H2O-RCT-CP replaceable cartridge countertop uses a proprietary cartridge size to ensure repeat sales. The GAC/KDF cartridge is designed to fit this housing and is retrofitted to work with standard housings. The housing only accepts H2O Intl. cartridges. It has interchangeable pre-filter disks, preventing early clogging and is available with Eco-Carb and threaded ceramic candle cartridges.