Before reading this description it may be useful to read ‘Aquarium pH.’ This can be accessed by clicking on Articles (top of page), then scrolling down to the section named Water Quality.
Initially it should be stated that the aquarist has no need to delve into ORP unless there is a wish to, or the aquarist has advanced enough to believe that understanding will be of value.
The Redox Potential (RP) can be measured by means of a probe, and is measured in millivolts (mV). (The pH can also be measured likewise in mV.) The RP and pH of an aquarium are interdependent.
At RP 300 (pH 8.2) the seawater is considered to be moderately polluted. The RP relates to the pollution load and pH. At a pH of 8.2, if the seawater is polluted, the RP could measure less than +100mV.