Higher CO2 concentration = lower pH. So adding CO2 to your tank would help your pH problem.
This is why you have to let tap water sit out for at least 24 hours before testing its pH. If you test it right away, the CO2 concentration is at its max. If you let the water sit for 24 hours and disturb the surface, the gas exchange will increase the oxygen concentration in the water an decrease the CO2 concentration. This causes the pH to increase.
If you look under the Article tab above and then in the freshwater section, I wrote an article on maintaining a proper pH. It talks about this in that article.