The two most important elements to supplement in your reef aquarium are calcium and alkalinity. If your reef aquarium is doing well, then you will need to supplement calcium and alkalinity. We have dozens of reef aquarium calcium & kalkwasser additives from Seachem, Brightwell Aquatics (Calcion / Reef Code), Two Little Fishies C-Balance, & Sea-Lab; some of these additives are two-part supplements for both alkalinity and calcium. Related Articles: Kalkwasser OverviewIFour Ways to Supplement Calcium and Alkalinity

From the Four Ways to Supplement Calcium and Alkalinity Article:
There are many additives and trace elements for reef aquariums, but as the rule goes—20% of the work gets you 80% of the results. The two most important elements to supplement in your reef aquarium are reef aquarium calcium and alkalinity. If your reef aquarium is doing well, then you will need to supplement calcium and alkalinity. These two elements are what make up the vast majority of coral skeletons, clam shells, crustacean shells, and snail shells. As these animals grow in your tank, they will suck calcium and alkalinity out of the water to build their bodies. If you don’t add any back in, then they will ... continued