Quote Originally Posted by Chabnoc
My last grow. I pretty much only had to ph my res when I replaced the water/nuts.
If your tapwater is 'hard' or has a lot of dissolved minerals, often limestone, this will cause the pH to rise. Nutrients also often include pH buffers, which will hold pH steady for a few days but will react with other things in tapwater and become less effective over the life of your tank of nute solution.

You can eliminate that problem by using distilled, RO or other demineralised water- or you can just live with the tapwater and just know it will need to be pH corrected.