There seems to be a very common misconception that aragonite sand helps buffer your water when it dissolves. While this is true if it's used in freshwater systems where the pH is typically around 7, it doesn't normally happen in saltwater tanks. Aragonite is calcium carbonate. It's the same thing live rock is made of and the same thing that makes up the skeletons of hard corals. In order for calcium carbonate to start to dissolve, the pH needs to be around 7.3 or lower. You don't see a pH that low in a typical saltwater tank. Where you may see the pH that low is very deep (6" or more) within a deep sand bed, but the area would be so small and confined that any dissolution of the aragonite would precipitate back into calcium carbonate because of the locally high concentration and not buffer the tank water. Now if aragonite did dissolve and provide buffers under normal water parameters with a pH around 8, not only would the aragonite sand dissolve, so would your live rock and any live hard corals with calcium carbonate skeletons.
