Redox or ORP measures the net balance between oxidizers (O2, ozone) and reducers (organics, ammonia). A higher reading means cleaner water with more dissolved oxygen and fewer pollutants. Reef target: 300-400 mV. Freshwater target: 200-300 mV. Below 200 mV: high organic load, low O2, often correlates with cyanobacteria + dinoflagellate blooms. Adjusting: ozone reactor (advanced), more skimming, water changes, reduced feeding. ORP probes (Pinpoint, Apex) drift over time and need calibration with quinhydrone solution every 3-6 months. Many reefers track redox as a leading indicator of water-quality crashes.
Reviewed by the Fast Aquatics husbandry team · Updated May 2026
Full glossary · Q&A library · Calculators · Disease database · Care guides