RO/DI Water for Aquariums
How reverse osmosis and deionization purify water, when aquarists need it, what TDS measures, and why pure RO water must be remineralized before use.
What RO/DI water is
Reverse osmosis (RO) uses applied pressure to force water through a semi-permeable membrane against the natural direction of osmosis. The membrane passes water molecules while rejecting larger species, removing dissolved salts, ions, suspended particles, and most bacteria. Deionization (DI) is an additional resin stage that strips remaining ionic content to produce near-zero-mineral water.
Membrane performance
Rejection depends on membrane type. Cellulose triacetate (CTA) membranes reject roughly 85 to 95 percent of dissolved solids, while thin-film composite (TFC) membranes reject about 95 to 98 percent. Freshwater and brackish systems typically operate at 2 to 17 bar of pressure.
Understanding TDS
Total dissolved solids (TDS) is the combined content of inorganic and organic substances dissolved in water, expressed in parts per million (ppm) or milligrams per litre (mg/L). It is most often estimated from electrical conductivity, which is accurate to about 10 percent. Common constituents include calcium, sodium, potassium, chloride, nitrates, and phosphates.
When aquarists need RO/DI
RO/DI is practically essential for marine aquariums, where mineral-free water is the starting point for mixing synthetic sea salt. In freshwater, it is used when tap water is too hard or too high in TDS for the intended livestock, or to create soft, low-mineral water for blackwater species and many shrimp that need it. An efficient RO unit can remove 90 percent or more of tap-water contaminants; well-purified product water should read around 10 ppm TDS or less with a general hardness of zero. The unit also produces a waste stream with elevated nitrates, phosphates, and heavy metals, which should not be used in the aquarium.
Why pure RO water is not used straight
Neat RO water is nearly mineral-free and has zero carbonate hardness, so it lacks buffering capacity and is prone to drastic pH swings. The absence of minerals disrupts the osmoregulation of fish and provides a poor environment for plant growth because essential trace elements are missing. Used directly, it can be lethal to fish.
Remineralizing before use
RO water is treated as a blank canvas: a remineralizing product is added to raise general hardness and, where needed, carbonate hardness to target levels for the intended species. Many aquarists blend RO water with a measured amount of tap water, or dose dedicated salts, to reach a stable, repeatable parameter set.