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Calcium reactors are devices used to maintain both calcium levels and alkalinity in marine aquaria. Although they look kind-of complicated, their principle of operation is quite simple.
They work by circulating acidic water through crushed coral or some other calcium carbonate media and dripping the resultant solution into a sump or display aquarium.
Crushed coral is predominantly calcium carbonate plus trace elements that are present in the same ratios that corals take from the water. Crushed coral, however, does not readily dissolve at the pH at which we maintain our aquaria. This is fortunate for our live corals; it would not be much good if their skeletons started to dissolve under normal circumstances.
If the pH is dropped to below 7, however, crushed coral will dissolve, liberating calcium ions (calcium maintenance), carbonate ions (alkalinity maintenance) and trace elements including strontium and molybdenum.
The question arrises as to how to exploit the dissolution of crushed coral at low pH, while maintaining safe pH in the main aquarium. That's where the calcium reactor comes in.
Calcium reactors inject carbon dioxide (CO2) into water drawn from the aquarium or sump. The CO2 "reacts" with the water to form carbonic acid, lowering the pH to below 6. This acidic water then dissolves the crushed coral in the reaction chamber and eventually becomes saturated with calcium carbonate. This water (the effluent) is returned to the aquarium or sump at a very slow rate, typically measured in drips per second.
Calcium reactors may be single pass or recirculating. In a recirculating calcium reactor, a pump circulates the water through the reaction chamber over and over. Only a very small percentage of water in any given pass is returned to the sump or aquarium. In contrast, a single pass reactor passes the water through the reaction chamber only once, albeit, very slowly. There are reasonable arguments as to why a single pass reactor is better, but most commercial models are recirculating.
The main advantage of a calcium reactor is constant maintenance of alkalinity and calcium levels, negating the need to regularly dose kalkwasser or two part supplements.
There are also problems that can be caused by misuse. The main problems are lowered aquarium pH and faster algae growth. Both can be attributed to the over-injection of CO2 into the reactor or bad reactor design.
Bubble counter: this is a simple device (usually just a clear tube) that is filled with water and allows the monitoring of CO2 injection rate by counting bubbles per minute.
Reaction chamber: this is the main chamber of the calcium reactor that holds the crushed coral media. It is where the calcium carbonate is dissolved.