As you may be aware, much is said of balanced additives but less often is this defined.
Before discussing that, what we need to know is why do we have to add calcium and alkalinity.
In the perfect world (and a mature tank), only calcium and carbonate are removed to form the corals skeleton which is pretty much pure calcium carbonate (call it chalk or marble if you like). However a few additional reactions occur so, for example the coral may also consist of magnesium carbonate and strontium, and calcium phosphate may precipitate out of solution (drop out the water). But as a good approximation the only places that calcium and alkalinity go is into corals skeleton.
Except (there had to be one!) if your tank produces nitrates. Its a bit of a weird equation but the nitrification process (that is the move from ammonium through to nitrate) removes alkalinity, whilst denitrification (nitrate to nitrogen) returns that alkalinity. You can compensate for this by adding one of the many buffer additives but long term you will be adding excessive sodium (these buffers are simply sodium carbonate, sodium hydrogen carbonate and often sodium borate). Yet another reason to try to achieve zero nitrates!
Coming back to this idea of adding excess sodium leads us on to the concept of balanced additions. Since the only thing being removed (in gross terms) is calcium carbonate, the only thing we want to add is calcium carbonate, otherwise, over time, the water in your tank will build up variations of the various chemicals away from that found in natural seawater.
So the question is, what is a balanced additive?
The most obvious one is via the use of a calcium carbonate reactor. That's one where we fill a sealed tube with calcium carbonate, recirculate saltwater within the tube (but slowly replace the water in the tube with water from the tank) and add enough carbon dioxide to lower the pH enough for the calcium carbonate to dissolve. Actually the output from the reactor is mainly calcium hydrogen carbonate but in the main tank with the pH in the eights that quickly becomes mainly calcium ions and carbonate ions (and water).
2 (HCO3) <-> CO3 + H20
This method relies on the fact that the hydroxide ion can pick up carbon dioxide to form carbonate to again give us calcium and carbonate ions in solution
CaOH2 + CO2 -> Ca2+ + CO3
These are the likes of Two Little Fishes' C-Balance, ESV's B-Ionic, Kent's Tectra A & B.
Bob Stark of ESV came up with the original two parter. The theory is cunningly simple, doing it well is more tricky.
Traditionally the way to maintain calcium and alkalinity was simply to add calcium chloride as one solution (or powder) and the various sodium based alkalinity chemicals as another solution or powder. Great, we've just added lots of calcium and carbonate but we are left with a lot of sodium chloride, which is bad.
However, saltwater is mainly sodium chloride, so what Bob did was add the rest of the chemicals that you find in saltwater into either of the two bottles of B-Ionic. This is where the chemistry gets rather clever as what he had to do is look at all the various ions, and find a combination of chemicals that contained those ions but (and it's a big but) make sure that all the ions stayed in solution. To see what I mean just get some water, some calcium chloride and some buffer powder and mix ... you will find you get a white powder that just doesn't dissolve (this is calcium carbonate, which is there if is enough calcium and carbonate, around would much rather be a solid and not in solution.
So what you are effectively doing is adding your artificial salt of choice (I mean the likes of Instant Ocean, Tropic Marin, etc, etc) but one that has much higher levels of calcium and carbonate than normal. The reason that you can't really buy it in powdered form is that the mixture would continually be picking up water from the air and forming insoluble calcium carbonate.
This seems to be popular in Germany but I have not seen it elsewhere.
This is a clever variation on the two parter method. If relies on having sodium chloride free salt (which Tropic Marin, for example, makes). Remember above where the goal was to just add highly enriched artificial salt ... well this is the same. Get your sodium chloride free salt, get lots of sodium carbonate and lots of calcium chloride. Add to the tank in the correct ratio such that the sodium and chloride added is the same percentage as natural seawater.
This even works for magnesium addition, instead of adding all calcium chloride, some or all of it can be magnesium chloride.
Really rather clever, a little more work than the two parters, but potentially a lot cheaper because rather than buying powder dissolved in water as you have to with the two parters, you are only buying the powder so the shipping costs are less.