# RO water

> RO water is water stripped of most dissolved salts by reverse osmosis; it has near-zero EC and little buffering, so pH moves easily and Ca and Mg must be added.

- Canonical: https://genetixbay.com/knowledge/glossary/ro-water.html
- Also written: reverse osmosis water, reverse osmosis, RO, RO filter, RO system, purified water
- Glossary category: pH, EC and water · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Reverse osmosis (RO) forces water under pressure through a semi-permeable membrane that rejects most dissolved ions and many organic compounds. Typical rejection is around 90–99% of [dissolved solids](https://genetixbay.com/knowledge/glossary/tds.md), depending on membrane condition and pressure. Systems usually include sediment and carbon pre-filters; the carbon stage protects the membrane from chlorine, and catalytic carbon is needed for chloramine.

## In practice

RO gives a clean, predictable base, typically below about 0.02–0.05 mS/cm (20–50 µS/cm). That is ideal for hydro and for growers with hard, high-sodium or high-alkalinity [tap water](https://genetixbay.com/knowledge/glossary/tap-water.md).

Trade-offs: no calcium or magnesium, so the nutrient (or a [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md) supplement) must supply all of it; almost no [alkalinity](https://genetixbay.com/knowledge/glossary/alkalinity.md), so [pH](https://genetixbay.com/knowledge/glossary/ph.md) drifts more and [pH Down](https://genetixbay.com/knowledge/glossary/ph-down.md) overshoots easily; and a wastewater stream, often 1–4 litres per litre produced. Some growers blend RO with tap water to keep a little buffering. Replace pre-filters on schedule and watch [output EC](https://genetixbay.com/knowledge/glossary/runoff-ec.md): a rising reading means a worn membrane.

## Don't confuse with

- **Distilled water**: purified by boiling and condensing; similar result.
- **Filtered tap water**: carbon or sediment filters do not remove dissolved salts.

## Myth check

RO water is a genuine reason a grow needs Cal-Mag. [LED](https://genetixbay.com/knowledge/glossary/led.md) lighting is not: the LED claim is a *Myth*. See [Cal-Mag](https://genetixbay.com/knowledge/nutrients/cal-mag.md).

## Related terms

[Tap water](https://genetixbay.com/knowledge/glossary/tap-water.md), [General hardness](https://genetixbay.com/knowledge/glossary/general-hardness.md), [Buffering capacity](https://genetixbay.com/knowledge/glossary/buffering-capacity.md), [Cal-Mag](https://genetixbay.com/knowledge/glossary/cal-mag.md), [µS/cm](https://genetixbay.com/knowledge/glossary/us-cm.md)

## Read more

- [Water quality for cannabis: RO vs tap, alkalinity, bicarbonates and chloramine](https://genetixbay.com/knowledge/ph-ec/water-quality.md)
- [Cal-Mag: what it is, when you need it and when you don't](https://genetixbay.com/knowledge/nutrients/cal-mag.md)

## Sources

1. Prasad L, Meehan M. Using electrical conductivity and total dissolved solids meters to field test water quality. NDSU Extension WQ1923 (rev. 2025). https://www.ndsu.edu/agriculture/extension/publications/using-electrical-conductivity-and-total-dissolved-solids-meters-field-test-water-quality
2. Sonneveld C, Voogt W. 2009. Plant Nutrition of Greenhouse Crops. Springer. https://doi.org/10.1007/978-90-481-2532-6