What to measure
| Point | What it tells you | What to do with the result |
|---|---|---|
| Feed water | Your starting dissolved-solids load | Record it monthly; a rising value can change DI consumption |
| Post-RO / pre-DI | How well the membrane is rejecting dissolved solids | Calculate rejection and inspect pressure, temperature and membrane age if it worsens |
| Post-DI | Whether the final polish is still working | Use 0 ppm as the practical change trigger; replace or troubleshoot when it rises |
Rejection calculation
((feed TDS − post-RO TDS) ÷ feed TDS) × 100 gives the membrane rejection percentage. With 300 ppm feed and 9 ppm post-RO, rejection is 97%. Compare the trend against your membrane's specification and your own historical readings; do not use the DI reading to judge the RO membrane because the resin can hide a failing membrane for a while.
What 0 TDS does—and does not—mean
DI resin removes ionised impurities that remain after RO, which is why a final 0 ppm reading is useful before mixing salt. But TDS is an aggregate conductivity-based signal. Red Sea's product FAQ notes that nitrate and phosphate may still be detectable even when TDS is very low, so a meter cannot replace good maintenance, source-water investigation or tank testing when the aquarium behaves unexpectedly.
Common patterns
- Feed high, post-RO normal, post-DI 0: the system may be working; expect more DI consumption.
- Post-RO suddenly high: check pressure, temperature, flush routine, membrane seating and the pre-filters.
- Post-DI above 0: the resin may be exhausted, channelled or installed incorrectly.
- Readings jump around: flush the first output, rinse the probe and confirm the meter is calibrated and measuring a flowing sample consistently.
Record-keeping that actually helps
Keep a small note with date, feed TDS, post-RO TDS, post-DI TDS, water temperature, pressure if available and the date each cartridge was changed. That turns “the water seems off” into a traceable maintenance decision.
Sources and limitations
The three-probe monitoring approach is consistent with the way SpectraPure describes feed, RO and DI monitoring. The caution about low TDS not being a complete contaminant test follows Red Sea's salt FAQ. This is an operating guide, not a substitute for a laboratory water analysis.