Check your tanks salinity first befor you adjust Ca/Alk/Mg. In case of deviations adjust salinity first, because it directly influences also concentraiton of Ca, Mg and Alkalinity/Carbonate hardness.
There are several option to adjust Ca/Alk/Mg.
The most recommendable, especially in terms of flexibility , is the so called
Each of the three parameters (Ca/Alk/Mg) can be raised to a specified level by adding a special chemical salt. This salts are also called Balling-salts and are available in good aquaristic stores and online shops. You can add them “dry” or “soluted in RO water”. Everything from “very price orientaded” to “fully automated” is possible.
Aqua-Calculator does all needed (chemical) calculations for you, as well as it helps you with information about needed working steps.
„KH Plus“, „Ca Plus“, „Triple Buffer“ & Co are nearly always identical to the above mentioned Balling salts , but nicely packed , with instructions for use, for a pretty high price. Some Premium-Sellers (eg.Tropic Marin, Fauna Marin) add further substances to improve ease of use or differentiate from (cheaper) competition.
„Balling Liquid Sets“ are nothing else than Balling salts soluted in purified water in a certain, sometimes even not accurately known, concentration.
This is acceptable for smaller tanks. For bigger tanks usage of the Balling Method/salts as above is clearly he favourable option, especially price wise.
Are also a good option, but different to the Balling method ratio between Calcium to Alkalinity can’t be adjust according tanks consumption. Raising also Magnesium concentration can be done by adding a certain amount of Mg-granulate to the reactors standard content which is Calcium-Carbonate -granulate.
Calcium reactors are good for bigger tanks, due they can be highly automated. Smoothly running reactor for a longer time, should also have an internal ph-Controller that control “ph” by adding “CO2” and a dosing pump (peristaltic pump) to “push” through the needed amount of water to the reactor. (exactly this amount of water will carry the additional Ca/Alk/Mg).Fully fledged Calcium reactors are relatively cost intensive.
Especially for tanks with nicely growing SPS, water changes would be needed very often and to a high %-age of your water volume, to support invertebrates with enough Ca/Alk/Mg. This is not only pretty expensive, also you can adjust your targeted parameters only by selection of the salt mixtures only!
Recommendation: Not the best option and pricewise only for smaller tanks.
Very cheap and being the preferred option some years ago, lime water is not any more recommend because it holds the risk of generating Phosphate depots within your tank! By adding lime water you also cannot adjust Calcium vs Alkalinity, and also can’t raise Mg-concentration.