Many reef invertebrates need calcium in their water to build their shells and exoskeletons. In a reef aquarium, corals and other organisms will absorb enough calcium from the water column to deplete it, so you must add supplemental calcium. In order to keep a reef aquarium healthy, you will have to monitor and maintain the correct calcium levels.
In the reef aquarium, calcium is measured in terms of milligrams per liter, or mg/L. You will want to keep calcium between 400 and 450 mg/L. Lower levels of calcium will not be sufficient for reef invertebrates to maintain healthy exoskeletons and shells. If the levels get too high, the calcium can cause the pH to rise to dangerous levels, since most forms of calcium are alkaline. Most marine organisms do not adapt well to changes in pH, or a pH beyond their comfort zone.
Monitoring Calcium Levels
If you have a reef aquarium, testing for calcium is not optional. You can purchase test kits at most high-end pet shops or online. Test weekly when setting up your aquarium, then monthly once the tank becomes established. However, if something else goes wrong in the tank—for example, if there's a change in pH—you should go ahead and check your calcium levels to be on the safe side.
Signs of Low Calcium
If your calcium levels dip too low, the first symptoms will probably appear in your invertebrates. All kinds of corals need calcium. Even "soft" corals needs some calcium, since their bodies contain calcium support structures—though they use less than hard coral. Without enough calcium, corals will grow more slowly than they normally do. Even slow-growing species will show symptoms like shrinking back and discoloration. If you see these symptoms in your coral, you should immediately check your calcium levels.
Raising Calcium Levels
Several methods of adding calcium to the aquarium are possible. First, you can add kalkwasser in powdered or liquid form. You should not add this to the main aquarium; instead add it to a sump or external filter if you have one. This kind of supplement can raise the pH if you add too much too quickly. You can also invest in a calcium reactor, which uses carbon dioxide to dissolve calcium safely. Lastly, using calcium-rich substrate and rockwork, like tufa rock and crushed coral sand, will help buoy the pH and calcium levels.