If you have to ask you probably don't need one.
People with high calcium demand, those with many SPS corals for example, find it difficult and expensive to keep up with that demand. Calcium must be supplemented to replace that used by the corals. There are three common ways to do that.
1. Dose calcium directly with a calcium supplement, single, two part, powder like Randy's formula (reef chemistry forum)
2. Mix pickling lime with fresh water and drip or dose in the aquarium (kalkwasser)
or 3. use a calcium reactor.
Calcium (along with strontium, magnesium and other stuff) is taken up by reef building corals to make their hard skeletons that you are familiar with. These skeleton are crushed to make crushed coral or reactor media. The calcium, if exposed to a low pH will be released back into solution where corals can use it again kind of a recycling thing. The trick is to get salt water, which is normal above pH 8.0 down to an acidic pH so this can occur. This is done by injecting carbon dioxide into the water. The reactor may be slightly pressurized and hold the CO2 in the water and then the water is circulated through the crushed coral. That water is saturated with calcium. Water from the aquarium is pumped into the reactor and an equal amount of calcium solution is sent out the other end. Then you need only dial in the amount of water needed to maintain the level of calcium you need. This provides a steady source of calcium for weeks or months without the need for daily dosing.
But like I said, if you do not have a high calcium demand then you really can't justify the expense.
Mike
