pH: A stable pH is very important in maintaining a healthy and stress free reef community. As mentioned above, a daily pH shift of +/-0.2 is normal in marine aquaria. The pH of Ocean water is 8-8.3, and marine aquariums should maintain a pH somewhere between 7.8 and 8.5.
In depth: “Several factors make monitoring a marine aquarium's pH level important. One is that aquatic organisms thrive only in a particular pH range, which varies from organism to organism. It is therefore difficult to justify a claim that a particular pH range is "optimal" in an aquarium housing many species. Even natural seawater's pH (8.0 to 8.3) may be suboptimal for some of its creatures, but it was recognized more than eighty years ago that pH levels different from natural seawater (down to 7.3, for example) are stressful to fish.6 Additional information now exists about optimal pH ranges for many organisms, but the data are woefully inadequate to allow aquarists to optimize pH for most organisms which interest them.7-11
Additionally, pH's effect on organisms can be direct, or indirect. The toxicity of metals such as copper and nickel to some aquarium organisms, such as mysids and amphipods,12 is known to vary with pH Consequently the acceptable pH range of one aquarium may differ from another aquarium's, even if they contain the same organisms, but have different concentrations of metals.
Changes in pH nevertheless do substantially impact some fundamental processes taking place in many marine organisms. One of these fundamental processes is calcification, or deposition of calcium carbonate skeletons, which is known to depend on pH, dropping as pH falls.13 …The acceptable pH range for reef aquaria is an opinion rather than a clearly delineated fact, and will certainly vary with the opinion's provider..”
Additional Sources: Brightwell Aquatics and www.ReefKeeping.com