ORP - Redox potential

The redox potential indicates whether the water has more oxidizing or reducing properties. The redox potential - also called ORP - is measured with a redox electrode and a suitable measuring device. The potential is expressed in mV (millivolts). The ORP depends on many factors: Oxygen concentration, temperature, pH, concentration of organic substances, among others. It is therefore a so-called sum parameter.

The ORP electrode should always have a clean platinum surface. Fouling drastically changes the measured value (too little is displayed). For precise measurements, the platinum surface should be wiped daily. The redox electrode also needs a longer time until a realistic value is displayed. Therefore, a continuous measurement should always be carried out. The redox electrode should be installed in a well flowed place in the aquarium or filter system.

The ORP measurement is not very simple; the measured value depends on the device used and the electrode. Thus, no absolute statements can be made about the measured value. Defective electrodes can indicate any value.
However, the trend of the redox measurement shows very well the development of an aquarium or filter. If the value in the aquarium increases, it is a sign that the water contains less organic substances, i.e. it is cleaner. Too high ORP values in the aquarium can only occur when ozone is added. Too low ORPs can be a sign of blocked filters or rotten soils.

ORP ranges (apply only to platinum electrodes):
Below -200 mV: toxic hydrogen sulfide is formed (in nitrate filters or in unmaintained soil layers).
-200...-50 mV: ideal in denitrification filters (nitrate filters)
200...300 mV: ideal for freshwater aquariums
300...400 mV: ideal for reef aquariums
greater than 400 mV: overdosing of ozone may occur
greater than 600 mV: in this order of magnitude water is disinfected