Alkalinity, also known as hardness
Similar to calcium, alkalinity also helps your Corals grow. Alkalinity levels are often a surrogate measure for bicarbonate, which corals convert into carbonate(for their calcium carbonate skeleton).
Aquariums that are not supplemented will experience rapid depletion of alkalinity, so it’s recommended to maintain your alkalinity levels at or slightly above normal seawater (7-11dKH). Alkalinity levels will also vary depending on the aquarists goals (for example, if rapid skeletal growth is desired than higher alkalinity levels should be targeted— cautiously, to avoid depressing the calcium levels and creating undesirable consequences).
“If left unchecked, the alkalinity drops below a critical level and pH begins to fluctuate more wildly than is normally suggested (a daily pH shift of +/-0.2 is normal in marine aquaria), causing stress to livestock. Alkalinity is also depleted by reef-building organisms during the production of aragonite (~60% carbonates by weight), and is therefore very important for their rapid growth. Maintaining the alkalinity in marine aquaria between 7 – 12 dKH (2.5 – 4.3 meq/L) will generally maintain pH within the desired range.” (Brightwell Alkalin 8.3 Powder)