This sais allot from the Reefkeeping article
"Most reef aquarists, aside from those that use ozone and must therefore monitor ORP to prevent overdosing, use ORP to monitor if anything unusual happens in the aquarium. A sudden drop in ORP, for example, suggests that the reducers are suddenly gaining ground. That might be because a gush of organic molecules has been released from a dead organism, or because the oxygen supply is not keeping up with demand for some reason. Aquarists might use such information like an alarm suggesting the tank needs to be looked at closely. Most aquarists do not target any specific ORP value as being optimal, in part because ORP measurement is subject to considerable potential error."