pH in the saltwater reef aquarium is the measurement of the water acidity or alkalinity level. This measurement is basically used as an indirect way to check for the level of dissolved carbon dioxide in the water. High level of dissolve carbon dioxide will cause the water to be more acidic and leads to stress and diseases among the fishes and invertebrates. The best pH value is at 8.3, which is the natural sea water value. However, due to the small volume of seawater in the tank compare to the vast ocean, this value will drift slightly through out the day with the value getting lowest during the early morning when carbon dioxide is produce but not remove during the night from the respiration of the tank inhabitants. For more information about pH and how to control it in the saltwater reef aquarium, click on the link.