The untreated hydroton can cause a pH spike.
Are you pre-mixing your nutes? You don't use a reservoir in a soil grow unless you are doing a drip on a timer but even then you have to be 100% certain that your nutes are stable in the res. I've just recently worked on a grow where the pH problem was actually being exacerbated by a grower who pre-mixed her nutes in gallon jugs and over time the pH was dropping.