In most cases a stable pH is much more important than constantly trying to adjust it, the regular pH swing will do more harm than good.
I would recommend testing the water for KH, this consists of bi-carbonate ions which form the buffering capacity of water. If the KH is low then this will usually result in a low pH, the opposite applies if the water has a high KH level.
Avoid using chemical pH adjusters, these contain hidden ingredients which can wreak havoc with your water quality.
Another thing to consider is the age of the aquarium, old tanks will naturally accumulate detritus. Fish waste is mildly acidic and may cause the pH to drop, however extra water changes should counteract this.
