im growing hydro...dwc....and noticed today that if i measure the ph from a sample of the water thats in my res. than the ph is around 5.2.....but if i measure ph directly in the res. the ph is much higher around 7.....is this unusual and which reading do i go by? Do u have any suggestions on how to lower and keep the ph down? i have been using ph down but seems like i can never keep the ph where it needs to be.