RO units (and RO/DI units) are dedicated water filters that strip out chlorine, chloramine, metals, nitrate, phosphate and other impurities from tap water, giving you a pure, blank canvas for mixing saltwater and top-ups. Using multiple stages of filtration (sediment, carbon, membrane and often DI resin), they help prevent nuisance algae, protect sensitive fish and corals, and keep your parameters far more stable than using untreated tap water. An RO unit quickly becomes an essential bit of kit for any serious marine keeper, saving money on shop-bought water and giving you full control over what goes into your aquarium.
