A calcium reactor is a device used in a reef aquarium to automatically replenish calcium, alkalinity, and other trace elements that are consumed by corals, invertebrates, and other reef inhabitants. It works by using carbon dioxide gas to dissolve calcium carbonate (CaCO3) media, which then releases calcium and other ions into the aquarium water.

There are several types of calcium reactors available in the market, but the most common types are:

1. Recirculating calcium reactor: This type of reactor recirculates the water from the aquarium through the reactor and back into the tank. The advantage of this type is that it provides better control of the pH and helps to maintain a stable environment.
2. Non-recirculating calcium reactor: This type of reactor only allows water to pass through the reactor once before it returns to the aquarium. The advantage of this type is that it requires less maintenance, but it may not be as effective as the recirculating type.
3. Dual-stage calcium reactor: This type of reactor consists of two chambers, one for the dissolution of the media and the other for the re-introduction of the effluent. The advantage of this type is that it provides better control over the pH and allows for a more efficient use of the media.

The use of a calcium reactor in a reef tank can provide several benefits, including:

1. A stable environment: The automatic replenishment of calcium and alkalinity helps to maintain a stable environment for corals and other reef inhabitants.
2. Reduced maintenance: A calcium reactor can reduce the need for frequent water changes, dosing, and other manual tasks.
3. Enhanced growth: Corals and other invertebrates will grow faster and healthier in a stable environment with adequate levels of calcium and alkalinity.
4. Cost-effective: While the initial cost of a calcium reactor may be high, it can be more cost-effective in the long run compared to other methods of calcium and alkalinity supplementation.

However, it is important to note that the use of a calcium reactor requires careful monitoring and maintenance to ensure proper operation and avoid adverse effects on the aquarium. It is recommended to consult with a knowledgeable aquarium professional before implementing a calcium reactor in your reef tank.