Reverse osmosis water is produced by a reverse osmosis system, which takes pressurized tap water and purifies it. Filtered water passes through one line, while waste water goes out another. A basic RO system includes two prefilters—a sediment filter and carbon filter—that remove most impurities. The TFC membrane then makes the water about 95% pure. A Ro water filter is good for freshwater aquariums.
An RODI unit adds a deionizing filter that makes the water 99% pure. RODI water is used by many reef aquarists. A TDS meter (total dissolved solids) measures water purity in the system. An auto shut off valve can be used to automate filling of a freshwater reservoir. Because a reverse osmosis filter needs pressurized water, low pressure houses, like those with well water, may need a water pressure booster pump.