You want the probe in the tank some distance from the CO2 reactor. That way it is reading more or less what the average tank PH is.
If it was inside the reactor, when the CO2 switched on the high Concentration of CO2 in the reactor would make the reading very low. Like suddenly jumping from 7 to 4 and then back to 7 when the CO2 switched off.
If you were going to put it inline with the reactor - put it before the reactor. That way you'll get the tank PH, not the reactor PH.
And my Otos don't seem to mess with my probes. I give the probes a a good rinsing every 4 months or so and compare the meter reading to my test kit every couple of weeks.
29G - ReefKeeper Lite Controller - Pressurized CO2 - Hydor Inline heater - Fluval 205 - Fluval Pimp #18