Medicineman:
Simply put, ORP (or redox potential) is the balance between oxidation and reduction, just as pH is the balance between acid and base.
If things are happening in the tank that consume excessive amounts of oxygen and other oxidizing substances (like bacterial degradation of a dead organism or reaction with dissolved organics), the ORP will tend to be lowered. If you are adding highly oxidizing compounds to the water (like ozone) the ORP will tend to be artificially high.
Unfortunately, there a tons of other things that impact OPR readings, making them not especially useful. Some people monitor ORP trends, and that has merit.
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Randy Holmes-Farley