ORP
I do not recommend that aquarists try to "control" ORP.
The oxidation reduction potential (ORP) of a marine aquarium is a measure of its water's relative oxidizing power. ORP has often been recommended to aquarists as an important water parameter, and some companies sell products (equipment and chemicals) designed to control ORP. Many who recommended ORP control have convinced aquarists that it is a measure of the aquarium water's relative "purity," despite this never having been demonstrated
ORP, at its heart, is very, very complicated. It is perhaps the single most complicated chemical feature of marine aquaria that aquarists will typically encounter. ORP involves many chemical details that are simply unknown, either for seawater or for aquaria. It involves processes that are not at equilibrium, and so are difficult to understand and predict. Even more daunting is the fact that the chemicals that control ORP in one aquarium might not even be the same chemicals that control ORP in another aquarium, or in natural seawater. In many seawater situations, the measured ORP value may be actually measuring the relative concentration of the different redox forms of various metals, such as iron and manganese.
ORP is, however, an interesting measure of the properties of water in a marine aquarium. It has uses for monitoring certain events in aquaria that impact ORP but may be otherwise hard to detect. These events could include immediate deaths of organisms, as well as long term increases in the levels of organic materials. Aquarists who monitor ORP, and who do other things that seem appropriate for maintaining an aquarium in response to the ORP value (such as increasing aeration, skimming, use of carbon, etc.) may find monitoring ORP to be a useful way to see progress.
ORP measurements are very susceptible to errors, however. Aquarists are strongly cautioned to not overemphasize absolute ORP readings, especially if they have not recently calibrated their ORP probe. Rather, ORP measurements are most useful when looking at changes in measured ORP over time.
Some aquarists use oxidizers (such a permanganate) to raise ORP, although this practice is far less common now than in the early days of reef keeping. These additions may benefit some aquaria, and may be beneficial in ways that aren't demonstrated by changes in ORP alone (for example, reducing the yellowness of the water). I've never added such materials directly to my aquarium, although I have used ozone in the past. In the absence of convincing data otherwise, such additions seem to me to be potentially riskier than is justified by their demonstrated and hypothesized benefits (except for properly used ozone).
ORP is important if you choose to use ozone as an indicator that you are not using too much and thereby putting your creatures at risk. In my opinion, however, the absolute ORP value is not even a good indicator that you are using an appropriate amount of ozone. For that purpose, the change in ORP from before to after the ozone use may be more appropriate.
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I would like to get a probe for it on my Apex however just for another graph to stare at. Wish they were a bit cheaper =\
Originally Posted by SaltWater
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