you must check your calcium and alkalinity readings.
ph,cal, and alk are all interelated. if you ph is low im willing to bet your alk is also low. alk directly buffers ph to help your tank from going through ph swings.
if you want a product that will take care of both for you try seachems 8.3 buffer. it will regulate your ph with regular use in your topoff water and help maintain your alk as it has a dose of that as well. do a search on the three and you will find info. look for broomer and the head honchos, they have discussed it many a time.