"As discussed above, a reef aquarium's pH rises when its water becomes deficient in carbon dioxide. In practice, this deficiency can be caused in several ways. The diurnal (daily) change in pH in reef aquaria occurs because of the biological processes of photosynthesis and respiration. Photosynthesis is the process whereby organisms convert carbon dioxide and water into carbohydrate and oxygen. The net reaction is:"


http://reefkeeping.com/issues/2005-03/rhf/index.php#6