If it was the air stones/aeration causing the CO2 changes, then levels would stabilize at equilibrium levels within 12-24 hours at most. So after the first day, there should be very little to no drift due to changing CO2 levels. It wouldnt start up again until after the next rez change.Using air stones creates carbonic acid as CO2 in the air bubbles reacts with your water. That's why the pH drops. If you run a RDWC with waterfall oxygenation you won't have that issue.
I dont see how water falls would or could be any different. Both cause basically the same amount of aeration and have the same exact effect on CO2 and O2 levels.
Aeration is aeration and CO2 concentrations will change or not change the same way no matter how you do it.
I agree, but in my experience, bacterial/algae infections generally push the PH up, not down.Hugh ph swings point towards bacterial infection.
he should check the rez for smell, slime, cloudy, etc and smell the roots. If that all looks good, then its something else - like a nute imbalance maybe.