I just found out a friend of mine, who uses Coralife salt, is having problems with his PH being low and not being able to get it up no matter how many water changes he makes (no jokes please).
FWIW, water changes are not usually a suitable way to raise pH, and the nature of the mix that you get is not going to cause low tank pH (unless the alkalinity is too low). Tank pH is determined by the carbonate alkalinity and the amount of CO2 in the water (which is largely determined by the amount of CO2 in the air).
These articles describe how to deal with pH issues:
Low pH: Causes and Cures
http://reefkeeping.com/issues/2004-09/rhf/index.htm
High pH: Causes and Cures
http://reefkeeping.com/issues/2005-03/rhf/index.htm