Do you use a sump tank? How far is your CO2 reactor output from ph probe? Do you notice your solenoid going on/off frequently(you hear a click sound every time it goes on/off)? Did you turn your CO2 bubble rate really high(so ph drops really fast when solenoid turns on)? Have you calibrated your ph controller recently, at least did a sanity check comparing to a test kit? I am seeing some warning signs here. It is time to fine tune your needle valve and turn it down a notch. I hope you use necessary precautions so that no CO2 disaster every happens. You probably have read the recent ph crash thread on the general forum on PT forums, it would be a devastating loss.
Out of all the possibilities, my best guess right now, is that your ph controller calibration is off, that is, ph is not as low as it shows. and you are not getting enough CO2.
FYI, in my 100gallon discus tank. kh=4, ph=6.7-6.8 with ph controller. ph=7.4 before I turn on solenoid with a timer(I shut if off at night). Takes 1.5 hours to reach target.
Hope to see you at the SFBAAPS plant swaps.
