I do use my pH controller, but the only problem would be that the probe would not be deep enough in the tank if I emptied half way, also my reactor is set up so that is stops working after the water goes below 75% full in the tank. I was planning on leaving the CO2 off when I did this change.
I think the pH would not just stick at 8.5 or above because the RO has organics in it (and yes, its KH checks at near 0; traces) which make it acidic (its pH is 6.4) and when you are adding the RO to the water, you are also taking away some of the KH of the tap water, I checked and its KH is near 7, which is still medium, but not near what it used to be... However, the pH may remain at 8.5, which is of little importance since the KH does go down.
My main concern is getting the KH low enough so when I add CO2 the pH drops below 7. At KH 8, the CO2 is 35 ppm when the pH is 6.9 and at KH 7 the CO2 is close to 31 ppm when the pH is again 6.9. Is 35 ppm too much for the fish? I know that 4-6 KH would be ideal, but I want to see how this works. Thanks!
Tank in transition! 55 gallons, hard water.