I describe the use of products such as Balling Part C and AF mineral Reef Salt here:
How a Two Part Alkalinity and Calcium System Works, and Why it Matters By Randy Holmes-Farley This article is intended to help aquarists understand some of the important differences between the great variety of two and three part alkalinity and...
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from it:
Ion Balance
There are three fundamentally different ways to dealing with the ion imbalances and reductions caused by the accumulating sodium and chloride and subsequent salinity adjustments. These are:
...
2. A second method involves using a third part intended to correct these ion imbalances. This method is technically simpler than #1 since one is not deciding what can go where.
Commercial approaches in this regard include
Tropic Marin Balling Part C and
Aquaforest Reef Mineral Salt as the third part. These are artificial sea salt mixes without the sodium chloride, and are seemingly easy for a salt mix manufacturer to make correctly. The reason such a system can work is that, as noted above, the accumulating sodium and chloride are in the same ratio as in sodium chloride. Sodium chloride is the biggest ingredient in any salt mix (it must be), but if it is left out, and combined with the accumulating sodium chloride from use of the alkalinity and calcium parts, one can have a residue that matches the original salt mix in all aspects.
These third parts work equally well whether the alkalinity part is sodium bicarbonate, or carbonate or hydroxide, since all add the same amount of sodium per unit of alkalinity added. They are thus easily combined with any other two part, such as a DIY, to take a hybrid approach to save costs or to use sodium hydroxide for the alkalinity part.
One minor point is that while conceptually this works out well, if one is doing water changes along the way, between when the excess sodium and chloride were added and when the third part is added and salinity is corrected, the final effect will not be perfect. I think this effect is quite minor unless one waits a long time before making the adjustments. Ideally, one would frequently be adding this third part. The amount to use will depend on how much of the other two parts are being used.
Another issue is that use of sodium chloride free salt cannot offset ANY consumption of ions, such as magnesium or trace elements, unless it is not actually sodium chloride free salt as both companies claim, and those ions may need to be added in some other fashion.
Finally, it is not clear to me whether these mixtures actually contain calcium or alkalinity. If they do, as they seeming claim from the description, that may limit how concentrated they can be made for dosing (due to calcium sulfate and carbonate precipitation), but my guess is they leave them out without telling folks. That issue does not really concern a user either way since they will be dosing and controlling calcium and alkalinity anyway.
There are DIY recipes for this part 3, and my
DIY two part recipes do include a third part. In my recipe, this third part is primarily designed to deal with magnesium and sulfate depletion, and is not focused on many other ions (such as bromide). They are cheaper, and many users have shown them adequate over the many years they have been used, especially when also doing water changes, but they are clearly less complete than the sodium chloride free salt mixes described above.
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