Seachem - Reef Buffer 250g
When used as directed Reef Buffer will raise the pH of your reef system to 8.3 without fear of accidentally overshooting the pH to a dangerous level. Whereas competing products simply use sodium bicarbonate, Reef Buffer is made with a blend of carbonate and bicarbonate salts. This blend of salts results in Reef Buffer yielding a pK of roughly 8.6 in saltwater.
The pK of a buffering agent is equivalent to the pH around which that buffer most strongly buffers. One would presume, then, that the pK of a saltwater buffer should be 8.3; however, the multitude of organic acids produced by the inhabitants of a reef system forces pH downward to a much greater degree than in a non-reef, marine system. A buffer with a pK of 8.3 would end up yielding a pH more in the range of 8.0-8.2. Reef Buffer's higher pK (8.6) allows for greater pH stability in a reef system where the bioload is significantly more than in a fish only system. Competing products are not designed specifically for the reef environment, so their pK is too low (in most cases, pK 8.3). At this pK, the proper pH can never be reached in saltwater.
Reef Buffer will also raise carbonate alkalinity; however, when pH is not an issue, Reef Builder or Reef Carbonate should be your first choice for a carbonate alkalinity supplement.