clcium reactor or kalk top offs are best, as they are balanced additives and do not dump in equimolar amounts of sodium and chloride to skew your conservative proportionality of the ASW mix.
Over time, the use of Bicarb supplements (NaHCO3) and calcium carbonate (CaCl2) for this purpose ultimately will skew the composition and proportion of the conservative elements, making it difficult to reach high supersaturation levels in closed systems for Ca and alk. Frequent 10% weekly to biweekly water changes reduce this effect, but it will still occur, requiring a once every 6 month's large (~50%) water change to restore the proportionality of the conservative elements to that of NSW. Loss of proportionality will prevent us from attaining supersaturation of the calcium and akainity desired by so many for hermatypic systems.
Ultimately in such a scenario, the system's salinity becomes a product of primarily sodium and chloride that are left behind when Ca and Bicarb are either spent to supply the buffer capacity for acid/base reactions, or for the formation of skeletons for the corals (which is the most prevalent will be determined by the biotope you're emuating). Preferably, we would be supplementing these substances to make up for the amounts of each used in the biogeological reef building purposes (coral skeletons), rather than driving up low pH that may be due to unrelated causes (like high pCO2 inside atmospherically tight houses...) by dumping increasing amounts of buffer into a system. It is not buffer that controls the pH, rather the ability of buffer to control CO2 levels in the system, so its consumption for this purpose is rather wasteful of supplementations and ultimately counterproductive as increasing amounts of Na+ are added in the form of bicarb.