Thanks guys for all the input
@Docedison, liutr:
I have thought about using one such turbulence pipe, but at the time I didn't have a pvc pipe handy for it and tried with half a water bottle, but I still had variance and scrapped the idea. However, a PET bottle and a PVC pipe certainly have different dielectric properties and at the time I didn't account for that. I will try it as it sounds like a very valid idea.
I am compensating for temperature changes with a LM35 sensor, whose value is being accounted for in the math for pH.
I have left any pumps and solenoids far away from the tank, to avoid such issues.