There's no reason to be concerned about ORP, especially during cycling.
About the only thing it is useful for, IMO, is noticing an unexpected death in the tank, and not overdosing ozone (if you use it).
This has more:
ORP and the Reef Aquarium - Reefkeeping.com
http://reefkeeping.com/issues/2003-12/rhf/feature/index.htm
from it (which is just two sections of the article):
ORP and water quality
Many aquarists have been lead to believe that ORP is a measure of water quality or purity. Manufacturers selling ozonizers and other oxidizers (like permanganate) have been especially keen to present that idea. But is it really true? Is a higher redox indicative of "purer water" even when that redox is manipulated artificially by adding strong oxidizers? Or is such an addition analogous to an air freshener that masks odors? I don't know the answer, but I think that aquarists should ask the question, and hope to hear useful answers before adding such materials to their aquaria.
Obviously, one can decrease the yellowing of water fairly quickly with oxidizers. It turns out, however, than many organic functional groups that provide color are just the ones that are readily oxidized. It is a common trick for organic chemists that need organic compounds to lack colored impurities to add an oxidizer that "kills off" the color in certain impurities, but leaves nearly all of the primary organic compounds behind. I've done it myself when making dyes for photographic film. You don't want the film to be yellow, so an oxidizer is added to the dye, let it oxidize the color away, and then use the unaffected dye in the film.
Of course, that decolorizing itself can be viewed as beneficial, but it is not necessarily indicative of the load of organics that have been removed from the solution. It is also not necessarily indicative of an improvement for tank inhabitants. The oxidizer did something to the organics. Maybe they are less toxic in the oxidized forms. Or maybe they are more toxic. Or perhaps they are not toxic regardless of the form. Maybe they are more readily metabolized by bacteria. Is that a benefit? The point is that assuming that such a treatment is of significant benefit to the aquarium may be in error.
If an oxidizer is added and ORP goes up in 30 seconds, is the water purer? Not likely. More likely, that addition shifted many of the redox species to their more oxidizing forms. Is that beneficial? Maybe. Is it detrimental? Maybe. For example, the bioavailability of certain metals may depend on the form that those metals take. Is increasing bioavailability of them desirable? It all depends on the details. Details that are simply not known for aquaria.
Perhaps continual use of ozone does help clear some organics from the water, and there is a long term benefit that may or may not be related to actual ORP readings that one gets from an aquarium. Is there data showing that to be the case, and then coupling that with some objective measure of benefit to the aquarium? Does that outweigh the potential concerns about the toxicity of reactive oxidants in aquaria? Again, I do not know the answer. Only careful studies with clear endpoints can give such an answer.
Recommendations for ORP
ORP is an interesting, if complicated, measure of the properties of water in a marine aquarium. It has uses in 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 that are monitoring ORP in an aquarium, and are doing things that otherwise 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, the most useful ways of using ORP involve looking at changes in measured ORP.
Some aquarists use oxidizers to raise ORP. Those additions may be of benefit in some aquaria, and they may be beneficial 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 have more potential risk than is justified by the demonstrated and hypothesized benefits.
