Essential to the Saltwater Hobby, Reverse Osmosis/De-Ionization Units turn hard tap water into safe & clean usuable aquarium water. Harmful substances such as Chlorine, Copper, Sulfates and other hard sediments can be harmful and even deadly to many marine species. Always use RO/DI water when creating fresh saltwater, top off water, or when rinsing and handling products that could contaminate your aquarium.