35 GPD Captive Purity RO/DI Four Stage System. The health of your reef starts with the source water. This was my first RO/DI unit for my old tank. Had to upgrade to a larger one because I got a much larger tank. If your not changing out more then 35gallons of water a day this is more then enough for you. Its in perfect working order I just tested it before I took the pictures to list.