I'm monitoring ORP. Why? I dunno. I am considering the use of ozone at some point, but I was really just curious.
As far as two pH probes goes, I'm going to have a total of three on my system. The first is just my regular tank pH, the second will be for the pH controller on my calcium reactor, and the third will be for my Apex to shut down the CO2 in case the pH controller fails.