Those aquarists that own a fish only system are not overly concerned with calcium levels as it is mainly the captive reef where the level needs monitoring.
Even an aquarist with a reef aquarium could be able to maintain a sufficient calcium presence by the use of commercially produced additives or even by routine seawater changes. The former options would usually apply to small aquariums and perhaps those containing soft corals only.
The larger aquarium, particularly one that houses SPS (small polyp stony) corals, is more than likely to find that routine seawater changes do not maintain a high enough calcium level. The use of commercial additives will maintain the level but the ongoing cost is likely to be prohibitive. So what’s to be done?
There are ways to automate the provision of calcium, and one of these is the calcium reactor. This works on a simple principle, and that is that if calcium rich media is placed in a chamber where the pH is low (slightly acidic) then the media will dissolve.
In the calcium reactor aquarium seawater is slowly moved through a chamber by a pump. Inside this chamber is the media. Also fed into the chamber is carbon dioxide, which reduces the pH causing the media to slowly dissolve. The seawater is then re-directed to the aquarium carrying with it the additional calcium.
The device needs to be a little more complicated as the carbon dioxide has to be fed into the chamber at a fairly precise rate which requires a control valve. Also, the output from the device, the seawater returning to the aquarium, needs to be controlled. These controls are necessary so that in the first place the media dissolves to a sufficient extent, and secondly so that the aquarist has control over the speed of delivery of the enriched seawater. This allows more precise control over the level of calcium in the aquarium seawater.
The trouble is, a good reliable calcium reactor is not a cheap device. (Is anything I hear someone cry?) If the aquarist is not sure of his/her DIY skills then the device is still worth the price as it saves time and labour with additives, in the long term probably costs less than using additives, and affords a better way of continuously feeding calcium to the aquarium to maintain the selected level.
If the aquarist is reasonably good at DIY, or knows of someone who is and who is willing to undertake projects out of interest, then a home built device is a definite possibility.
The link provided gives good detail on material requirements and procedures with lists, instructions and pictures. There are even suggestions where the materials could be sourced. If DIY is being considered, do not be initially put off by what appears to be fairly complicated – when studied for a while it isn’t.