Why it matters
ORP gives instant feedback on whether sanitisation is working. Drops in ORP signal high bioload before pathogens become a problem.
Ideal ranges
| Water type | Ideal | Acceptable |
|---|---|---|
| Reef | 350 to 450 mV | — |
| Pool | 650 to 750 mV | — |
| Spa | 650 to 850 mV | — |
- Pool: WHO standard for safe swimming is 650 mV minimum.
How to test
- 1
Calibrated ORP probe (Apex, GHL, Neptune, Pentair)
Calibrate every 3 months with 200 mV and 600 mV calibration fluid.
How to correct low orp
Pool: raise FC, lower CYA
Sanitiser is the largest driver of ORP.
Reef: ozone or activated carbon
Ozone raises ORP fast (over-dose risk). Carbon polishes water and lifts ORP gently.
Common causes
- •Heavy bioload
- •Low sanitiser
- •Algae bloom
- •High organics from fish waste or swimmers
FAQs
Why is my ORP probe drifting?
Probes need cleaning monthly. Soak in hydrochloric acid solution then recalibrate. Replace every 12 to 24 months.