First, figure out if you need a calcium additive. Most systems are better off if you replace calcium by doing water changes. Usually only systems that contain massive amounts of SPS corals and/or clams require a calcium supplement.
BTW, if you find that a newly mixed batch of SW is low in calcium, you need a new brand of salt.
If I had to adjust calcium one time, I'd use calcium chloride. It's not good to use often, since it can imbalance the water.
If I needed calcium on a regular basis, I'd use a calcium reactor.
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