Sure, it changes constantly. As water evaporates and nutes are eaten your pH as well as your ppm will vary. Your ppm goes down as plants eat and your pH usually goes up as plant waste degrades and salts, etc build up. The AG is such a smal tank (1gl.), it doesnt take much to change the pH or ppm. Any pH between 5.2 & 5.9 is good... 5.8 is perfect. Keep it on the acidic side to avoid root problems but not so low or high to lock out nutes..