Adding Co2 will actually drop the pH. Increase the surface agitation on the top of your tank to pull in more O2.
pH Simply Explained
pH (power of Hydrogen) is simply a measurement of the acidity or alkalinity of a solution. A pH of 7 is considered to be "neutral", neither acid or alkaline, while pH above 7 is alkaline or "base", and below 7 is acidic.
The normal trend for pH in a saltwater system is downward, or more acidic, which stems from the addition of acids into the aquarium. These acids come from several sources, the primary ones being: (1) excess carbon dioxide (CO2) from respiration caused by lack of sufficient gas exchange, (2) nitric acid from biological filtration (nitrification), and (3) organic acids from metabolic wastes.
Of course respiration and metabolic wastes are a natural part of the ocean, but the reason that sea water pH does not change is that the water contains a number of chemicals, such as bicarbonate, calcium, carbonate, borate and hydroxide, all of which act as natural "buffers" that reta rd the drop in pH.
So where does alkalinity come into all of this? The degree to which a solution maintains its pH when acid is added is termed the "alkalinity" of the solution. Related terms used in reference to aquariums are carbonate or calcium hardness, and its German equivalent, KH or dKH. The amount of "buffers" in sea water determines the alkalinity.
When the pH in a saltwater system starts to drop, it is an indication that the buffers are getting worn out, and the increase in acidity needs to be corrected. (1)
(1)
Aquarium pH Control For Dummies - Saltwater Aquarium pH and Alkalinity Simplified