I came across an article today that was so impressive and loaded with information that I feel I have to share it. Here is the link: Advanced Aquarist's Online Magazine - Aquarium Chemistry: The Carbonate System in the Aquarium, and the Ocean, Part III: Methods Available to the Aquarist.
For those of you, whom after reading this article, feel overwhelmed by how to apply this information to your aquariums, I will share the following conclusion which I have reached.
Every marine aquarium utilizes calcium, be it fish only, fish only with live rock, or reef. Calcium is an extremely important part of your buffer system and is needed for coraline algae growth. For this reason, calcium needs to be included in your testing routine.
Per the study referenced in this article, large weekly water changes do not
replenish the calcium which is used in a LOW usage aquarium environment, such as a fish only with live rock system. In an environment which utilizes high amounts of calcium, water changes are practically useless for the purpose of replenishing calcium. For this reason, calcium needs to be added directly to all marine aquariums
Per the study above, there are no substrates in the hobby which are capable of dissolving at a rate that is effective in replenishing the calcium needs of a marine aquarium. Specifically, relying on aragonite alone will only replenish 4% of the needed buffering capacity of a low calcium usage marine system.
I think most of the experienced marine keepers on this forum will support my end conclusion, which is to say that the testing of alkalinity and calcium are highly important for long term success in this hobby. And adding a supplement is the most effective way to ensure that these levels are maintained correctly.