the ph will constantly fluctuate , as the water level changes, and as the food is eaten by the plants
try not to fuss over it so much , always allow a good hour or so before you take any readings, allow for the solution to mix up after you add anything
your ph is fine if it is anywhere around 5.5-6.5
take readings no more than twice per week , slowly break yourself out of the habit of too much fussing over ph
peace