How are you measuring your pH? Unless its with a calibrated electronic meter, I would HIGHLY suspect your readings as this is pretty low. Under most conditions, most reefs with live rock will bottom out around 7.6-7.8. pH values lower will typically dissolve enough calcium carbonate to bring values back up. Be sure your pH is truly that low before making any attempts to correct.
If you can verify that it is that low, boosting alk will raise your pH. Be sure to go slow when correcting.