It depends more on your water. If your PH stays between 8.3 and 8.4, you don't need a buffer at all. Theoretically, once you establish your PH, your RO water (which is neutral) should take on the properties of the tank when you add fresh. Your synthetic salt will have a buffer that should be all you need. However, this is not always the case, especially for new tanks. For several years, my PH would run low without adding buffering. Once the tanks were well established, this was no longer necessary. If you do find you need to buffer, I recommend one of the brands that are designed to minimize the chance of over-buffering. So SeaChem's Marine would be my choice but I have not used SeaChem's Reef product. Reading the product blurb, SeaChem is directed at tanks whose inhabitants tend to lower the PH (corals) and is targeted for animals that you would not keep with an octopus. If you add a buffering component, add a small amount to either your top off water or your salt exchange water, never add the buffer directly to your aquarium,