There are many things that affect PH. Ambient CO2
, acids, poor gas exchange, fish respiration and on and on really.
Alkalinity is just as important though. It is a measure of the tanks ability to absorb acids and resist changes in PH.
In a newly set up tank low PH is normal depending on how it is being cycled, excessive dieing matter will produce acids as well as carbon dioxide. These acids act against the PH and pull it down.
PH should not be used as a guage to represent alkalinity, but alkalinity can be used as a "loose" guage for PH. That greatly depends on what is being used to maintain the alklinity however. There are many additives in todays buffers and other alkaline products designed to maintain PH while raising the alkalinity. In most cases if alkalinity is maintained properly and the tank has sufficient water flow and ample "surface area" for gas exchange, PH will most often follow and be maintained along with it.
In the beginning it is something I would not worry about while the tank is cycling. You will find that once things settle down, the reduced acids will allow the PH to come back up or at the very least be easily corrected.
For the meantime >>here
<< is a very good article that includes related link that should explain in great detail about the chem relationships.