I have the same issue with my reservoir. Every few days I have readjust my PH...I Don't think it is something you can control as nature finds a way to react with one another. If you are using organic nutrients, the oxygen help keep the beneficial bacteria alive but raise the PH due to growth of the bacteria.... If using soil, as long as you put in what is right, don't worry about runoff. I hope this helps, good luck and happy growing