Reverse osmosis (RO) water
Water purified by reverse osmosis. The basis for preparing synthetic seawater and for water changes in freshwater aquariums when pure water is needed.
Detailed explanation
Reverse osmosis water (RO water) is tap water passed through a semi-permeable membrane that removes 95-99% of the dissolved solids. What comes out is almost pure water: no chlorine, no heavy metals, no silicates, no phosphates.
In marine aquariums, osmosis water is essential. Tap water contains silicates that feed diatoms, phosphates that feed algae, chlorine that kills filter bacteria, and copper that kills invertebrates. All water used to prepare synthetic salt and to top up evaporation must be osmosis.
In tropical freshwater aquariums, osmosis water is needed when very specific parameters are desired. For discus, cardinal tetras and other soft water fish, osmosis water is mixed with a little tap water (or remineralised with special salts) to obtain the desired hardness.
A basic reverse osmosis system costs 50-100 euros and produces 100-200 litres per day. DI (deionization) stage systems produce water with zero TDS, perfect for the reef.
Waste is the main problem: for every litre of pure water, 3-4 of waste water are thrown away. This ratio improves with good quality membranes and the right water pressure.
The membranes must be replaced every 2-3 years. Sediment and activated carbon prefilters every 6-12 months. A dirty prefilter reduces the yield and quality of the water produced.
Pure osmosis water contains no buffer and has an unstable (often acidic) pH. In marine aquariums it must be mixed with synthetic salt which restores all the necessary minerals. In freshwater aquariums it must be remineralised with specific products before use.
Reference values by aquarium type
| Aquarium type | Recommended range |
|---|---|
| Marine (reef) | 0 - 5 TDS ppmIdeal 0 TDS with DI stage |
| Tropical freshwater | 0 - 10 TDS ppmIt is remineralised before use |
| Coldwater / pond | 0 - 10 TDS ppm |
| Brackish | 0 - 5 TDS ppm |
How it is measured
With a TDS metre. The water leaving the system must have TDS close to zero. If the TDS rises it's time to change the membrane or prefilters.
What happens when it is too high
High TDS leaving the system indicates exhausted membrane, dirty prefilters or insufficient water pressure.
Coralia Lab monitors this parameter with AI diagnostics. Tank Score alerts you when a value moves outside the recommended range for your aquarium.
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