I'm constantly fighting PH, my last grow had two tubs, same nutrients in each and fed from the same top off res. Long story short, each tub had its own ph issues (In tear down I noticed one of the stones was really blocked off by roots and that could of been the problem. At the end I just tried to keep them in the 5.8 range and accepted it was going to be constantly changing. However I still keep trying to find the root cause.