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Old 23-09-2007, 10:17 PM   #5
valerossi
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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 clearly 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.

ORP is an interesting, if complicated, 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 (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. 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 to raise ORP. These additions may benefit some aquaria, and maybe in ways that aren't demonstrated by changes in ORP alone. I've never added such materials to my aquarium. 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.

Summary

Chemical issues in reef aquaria are often daunting to aquarists. There are many chemical parameters that aquarists monitor, some of which are critical for success, and some of which are much less important. Of those critical for success, only calcium and alkalinity require regular supplementation in all reef aquaria, although the others in Table 1 may require monitoring. Successfully keeping the parameters in Table 1 at appropriate levels should go a long way toward allowing aquarists to more fully enjoy their aquaria while at the same time ensuring that the inhabitants are well cared for.
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