PH naturally swings as the plant is picking up what it wants. By going in and manipulating the pH with acid youre making the plant have to fight even harder to draw the nutrients it needs at that moment. Between 5.5 and 6.5 are safe zones and even higher 6s are common during flower. Unless you start with a 6.0 pH and come back and check and its like 6.6 or higher or 5.4 or lower leave it alone.