I hate doing this, but a general question about pH is best answered with some reading material. Give those a once over, they cover all the important chemical relationships, methods, problems etc. Low pH can be caused by a number of things and after reading these you should have some idea on what may be your particular issue and how to resolve it. The problem you are having could be related to chemistry balance of the tank itself or something environmental.
Here are some really good reef chemistry articles:
The ?How To? Guide to Reef Aquarium Chemistry for Beginners, Part 1: The Salt Water Itself
http://reefkeeping.com/issues/2007-03/rhf/index.php
The ?How To? Guide to Reef Aquarium Chemistry for Beginners, Part 2: What Chemicals Must be Supplemented
http://reefkeeping.com/issues/2007-04/rhf/index.php
The "How To" Guide to Reef Aquarium Chemistry for Beginners, Part 3: pH
http://reefkeeping.com/issues/2007-05/rhf/index.php
The ?How To? Guide to Reef Aquarium Chemistry for Beginners,
Part 4: What Chemicals May Detrimentally Accumulate
http://reefkeeping.com/issues/2007-09/rhf/index.php
http://www.reefkeeping.com/issues/2004-05/rhf/index.php
http://reefkeeping.com/issues/2005-11/rhf/index.php
http://www.reefkeeping.com/issues/2004-09/rhf/index.php
http://www.reefkeeping.com/issues/2005-03/rhf/index.php
Once you have read these, give us an better idea the tank itself. Things we need to know to help would be:
Size of the tank?
Water change procedure, how often, how much?
What salt do you use?
What your parms are in the tank:
Salinity
pH - how are you measuring it
Alkalinity
Calcium
Magnesium
Do you run skimmer?
Is the tank closed, ie a canopy?
What do you use to dose your Alkalinity, Calcium and Magnesium? How often and how much do you use? What brands etc?