Here's a useful article that uses a clever analogy as measuring ORP is like measuring who is winning the war between two battling sides, the oxidizers and the reducers. That, in a nutshell is how ORP works. For aquatic life to sustain, there always needs to be a battle between these two forces. If one wins out entirely, then your fish will die very quickly.
One the one side, you have the oxidizers. Ever leave the guacamole out, or see your pristine apple slice turn brown? That's the work of oxidizers. They take electrons from organic molecules as the initial processes of decomposition begin. Those organic molecules are the reducers. In fish tanks these are the food, fish poo, and dead fish.
So is ORP useful? To an extent, yes. Ideally fish farmers want to keep their ORP between 300 - 450mV. This is what the ORP of the ocean tends to be. This means the system is aerobic with bacteria breaking down the reducers without producing harmful compounds for fish. It's not as good of an indication of how your system is doing because it can fluctuate a lot and even the expensive probes aren't that response the latest events. Given your sensor is working responsively, it can be a good indication of how much excess food is in your water and can help limit overfeeding. But be careful because it can be hard to tell if that spike in ORP is because of excess food, or a dead fish decomposing.
Have any experience with ORP you'd like to share? Leave us know in comments.