There are many comments about ozone and measuring ORP on Reefcentral. Here's just one that talks about it.

hi. as you probably know, monitoring the redox potential "ORP", is a rule-of-thumb way to monitor water quality, mainly dissolved organic carbons levels or "DOC". there is no ORP magic number, but the generally accepted aquarium range is between 300 and 450 mv. the best use of ORP monitoring is that you can detect a rapid drop in the mv reading and that can tell you that something has died and is polluting the water rather quickly. you don't need to have a 450 mv reading to have a healthy tank. if it looks thriving at 300 mv, then that may be a good base reading for your tank to use as a reference point. the ORP will fluctuate through the day due to lighting.

ozone can be used to raise the ORP of the water by oxidizing DOC and other compounds in the skimmer. the problem with ozone is it breaks down other desirable compounds and trace elements as well as acting like a sterilizer to beneficial plankton. if to much gets to the livestock, that's a bad thing. flow the ozonated water over carbon before returning it to the tank, and use the minimum effective amount of it you can get away with.

oh yeah, if you're considering ozone, i would invest the dough in a higher performance skimmer before an ozonizer, unless you've already got one and just want an extra tool. a good quality skimmer is better money spent. jmh2c

i use rarely ozone for short term to scrub DOC if the ORP drops below, say 225-250. it's not something that i would say is a must have item unless you're curing batches of rock or something like that.