Rhizo really kicks up the pH. Pretty soon I'll be doing a second round with coco. From my experience and a couple of emails to Canna, its really best to prepare the rez the night before (unless you prepare and feed everything straight away)
Its also cool 'cos that leaves time for the chlorine to evaporate and the nute solution to stabilize. Whats your base pH? (I had fluctuation during my grow of pH 6 to pH 8 !!) That also factors in.
Next rez, dont pH until the next day and start taking notes. You'll see that you'll start using less acid (vitagrow is citric acid), so less chemical stress.
I was also bubbling my rez, which Canna does not advise. They say that yes, it does force oxygen exchange, but so pH rises over time. And also that it will have no significant effect, water temperature being far more important in retaining O2. So bubbling only really makes sense if directly applied to the root system. The release of chlorine also affects the mixture a bit, so what I started doing was to bubble the rez (just water) for a few hours to help evaporate chlorine, then prepare the mix, let it sit 12 to 24h, then pH correct... And many times pH correct wasn't needed at all !
Started having really stable pH mixes and even run-off started being more consistent