What is your tap waters PH? if its nearer 7 then you could be slipping on the tank maintenance, of course I know you wouldnt do THAT!!!
So if it is nearer 7 from the tap, small max 10% water changes every day will help. Also try and find out the hardness of your water, if its soft, the PH will slide alot faster so you need to keep an eye on it. I personally wouldnt recommend using any chemical or product to raise or lower the PH, if its 6.2 from the tap, which is unlikely I think as most water companies try to raise it to neutral to avoid corroding pipes, then consider species that thrive in this PH like some south american or west african species.
Ken