An excerpt from:
http://www.reefkeeping.com/issues/2004-05/rhf/index.php
http://reefkeeping.com/issues/2003-12/rhf/feature/index.htm
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.