Reverse osmosis and deionization water purification filters provide superior rejection of ionic, organic and particulate impurities and thus produce contaminant-free water. In reverse osmosis, the ro membrane acts like an extremely fine filter to create purified water from otherwise contaminated water. Starting with purified water eliminates the primary cause of reef tank algae problems by removing excess phosphates, silicates and other impurities. RO/DI Systems are designed to further purify the RO product water by passing it through a DI (deionization) cartridge which is integrated into the system. The DI cartridge contains ion exchange resins which remove the last traces of contaminants from the RO product water.