Boost your Alkalinity up with some baking soda and your pH will raise shortly after. Add only enough to increase it about 1 dkh at a time by using this calculator:
http://reef.diesyst.com/chemcalc/chemcalc.html
Also read this for a better understanding.
Low pH: Causes and Cures
http://www.reefkeeping.com/issues/2004-09/rhf/index.php