A calcium reactor may be described as a 'balanced' calcium / alkalinity additive. Basically, this means that it adds calcium and alkalinity to the tank in the same ratio as is used by our corals during the process of calcification. Simply put, it is not possible to change the calcium level without the alkalinity being affected also in a defined manner.
As an example, for each 1 mEq/L alkalinity (2.8 dKH) the calcium reactor adds 20ppm calcium. If your tank starts out with 3 mEq/L alkalinity (8.4 dKH) and 320 ppm calcium, and you raise the alkalinity to 4 mEq (11.2 dKH) using the calcium reactor, then the calcium level will only increase to 340 ppm!
Natural seawater at 35 ppt salinity typically has around 2.5 mEq/L alkalinity (7 dKH) and a calcium level of 410 ppm, but I personally aim for around 3 mEq/L alkalinity (8.4 dKH) and 420 ppm calcium, and many others prefer even higher levels. Once you have decided on the levels, it is a useful idea to map where the calcium and alkalinity levels are (Bingman 1998) and then perform any corrections needed to get them back on target.
If the calcium level needs boosting, then I recommend using an additive such as calcium chloride. One gram of an anhydrous calcium chloride product (such as Turbo Calcium) will raise the calcium level by 360 ppm in 1 litre of water (95 ppm in 1 gallon of water).
If the alkalinity level needs boosting, then sodium bicarbonate can be used. One gram will raise the alkalinity by 12 mEq/L (34 dKH) in 1 litre of water (3.2 mEq/L (9 dKH) in 1 gallon of water).
