This is because CO2 decomposes into carbonic acid, giving rise to a decrease in the water pH. This carbonic acid has the ability to decompose into carbonate and bicarbonate. Basically this is the natural buffering system of the ocean. Well the key is to understand what factors will cause the pH of your aquarium to rise or drop below the desired pH of about 8.2. Most of the time, pH will not rise in an aquarium. If it does, usually it will fall back to normal values soon after, and causes no major problems.
Extensive lighting can cause such an event.
The only time that this can be a problem is when one has inadequate filtration in their system. Fortunately pH usually only drops. Such cases cause a decrease in pH due to an increased CO2 concentration in the aquarium. Another cause of decreased pH is high organic acid and phosphate levels in the aquarium. These will tie up the carbonate and bicarbonate in the aquarium, and reduce the natural buffering system.