Our RO/DI units are for Reef/Aquarium enthusiasts who require only the purest of water and when reverse osmosis is not enough. Reverse osmosis removes a large majority of TDS (total dissolved solids) but not enough of it. TDS is can be lowered to extremely low levels by using a DI (deionization) filter. With our systems, you will typically get ZERO to near ZERO TDS readings!!! Additional Details ------------------------------ Product description: Our RO/DI units are for Reef/Aquarium enthusiasts who require only the purest of water and when reverse osmosis is not enough. Reverse osmosis removes a large majority of TDS (total dissolved solids) but not enough of it. TDS is can be lowered to extremely low levels by using a DI (deionization) filter. With our systems, you will typically get ZERO to near ZERO TDS readings!!!