Tap water often contains phosphate, silicate, nitrate, chloramine and metals. RO/DI removes nearly all of it, giving you a clean base for salt mix and top-off.
TDS targets
- After RO membrane: typically 2–10 ppm depending on source
- After DI resin: 0 ppm
Once DI output rises above 0–1 ppm, replace resin.
Replacement schedule (typical)
| Stage | Replace |
|---|---|
| Sediment | 3–6 months |
| Carbon block | 3–6 months, or sooner with chloramine |
| RO membrane | 2–3 years or when rejection falls |
| DI resin | When TDS rises above 0 |
Equipment: RO/DI system guide. Science: salinity and RO/DI.
FAQ
Is store-bought RO water fine? It can be, but test TDS; many shops lack DI.
Why algae after a new RO/DI? Check TDS; exhausted resin can leak phosphate and silicate.