Couple of points before answering the question.
What is your PH currently at? If it is between 7.9 and 8.5 that is an acceptable range. Also, if the PH is low it usually is a sign that your alkalinity is low as well. In which case, if the alkalinity gets buffered up to normal parameter then the PH evens itself out.
Having said that. If all other parameters are normal and PH is still low then can use Baking Soda to raise it. Have to be careful on which kind to get. There is a new brand of Baking Soda out there that has some other chemicals in it and is not good for the fish. Personally, I wouldnt use it anyway.
As a side note- Tapwater is usually the culprit for having low PH. When I first started my system I was using tapwater and the ph was way low. Like around 7.5 and it was real hard to get it to raise up. I switched to RO water and it stays pretty consistent now. So much so that I dont even check it anymore.