Aloha,
There isn't a cover on the tank is there? Is Alkalinity within range? You know, for the most part, pH follows Alkalinity. How about water flow? Is it rippling/tearing up the water surface? If the pH is is lower in the morning, maybe a refugium with a good growth of algae is needed. Sometimes the Refugium algae has stopped growing because the light is old. These are the things I would look at which might negate the need for a CO2 unit.
Mahalo,
Mark
Edited by Mark Peterson - December 21 2013 at 12:22pm