I believe I have a better description of what a two part does here:
There is no aspect of reef aquarium chemistry more important than calcium and alkalinity. Many of my previous articles have described various aspects of these systems in detail. In reading those articles, aquarists will note one pervasive theme: that maintaining appropriate levels of each are...
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from it:
Two-part Balanced Additive Systems
There are now a plethora of two-part balanced systems for supplementing calcium and alkalinity, as well as DIY recipes that I have published and for which suppliers sell quality DIY ingredients. These are always liquid additives that you add equally to tanks to supplement both calcium and alkalinity. In the DIY version, magnesium is added to the aquarium as a third solution, although it need not be added especially frequently. The rational for this type of product is that the bicarbonate and carbonate that one might like to dose to supplement alkalinity are not readily compatible with the calcium that is also needed. So one portion contains calcium and the other contains the alkalinity. When a DIY is used, the magnesium sulfate in it is not compatible with either part, so it needs its own solution.
In the simplest form, such a system would be provided by any calcium salt at one concentration in one bottle, and a carbonate alkalinity supplement in the other bottle. Within that constraint, manufacturers have a fair amount of room to play. Typically these additives claim go a step further. When the calcium and alkalinity are taken out of the picture, as they will be by calcification in the tank, then the ions that remain are often described as having the same ratios of ions as natural seawater. Assuming that this is true, then the “residue” is simply more salt for the aquarium. Over long periods of time the salinity will build up due to this process (an effect that is quantified below), but there will be no significant buildup of specific ions in the tank.
In order to accomplish this, manufacturers could use a variety of calcium salts in the calcium portion, for example. They could use calcium chloride, calcium sulfate, calcium bromide, and a variety of other similar salts. They could also put magnesium and strontium in this portion as they would not be compatible with the alkalinity component.
The alkalinity portion of these systems is more complicated. As has been shown in other parts of this article, alkalinity can be provided as bicarbonate, carbonate, or hydroxide. I don’t know of any commercial supplements that use hydroxide for a two part system, but the commercial ones do use bicarbonate, carbonate, and mixtures thereof. Consequently the pH varies substantially between brands, and the various brands of these products should not be thought of as identical for this reason, if no other. In order to attain the natural seawater residue, the alkalinity portion could contain sodium bicarbonate or carbonate, potassium bicarbonate or carbonate, lithium bicarbonate or carbonate, etc.
I’ve not seen any independent test of whether these actually produce a residue equivalent to natural seawater, but I’ve seen no particular reason to doubt it, at least for the major ions. When it comes to the trace elements that might concern some reef keepers, it seems unlikely that these products will be any less prone to having uncontrolled levels of trace compounds like copper than are commercial salt mixes, or any other supplement of calcium and alkalinity, but that remains to be determined (at least as far as I know).
One issue that has confused some reef keepers, however, is the presence of trace elements. Assuming that these products are actually formulated with every ion such that a true natural seawater residue remained (let’s call this the “ideal” product), then it will necessarily contain such ions as copper. Since copper is elevated in some reef tanks, and is toxic to many invertebrates, reef keepers have wrongly criticized this method as adding more copper. That’s actually not what would happen. Since these products leave a natural seawater residue, and since copper may be elevated in concentration in many reef tanks relative to seawater, then using these “ideal” products will actually LOWER copper levels because when the increase in salinity is corrected, the copper will drop.
For example:
You have copper in your aquarium at 4 ppb and salinity of S=35.
You add a two part additive that over the course of a month raises salinity to S=36, and raises copper to 4.02 ppb.
Then you correct the salinity back to S=35 by diluting everything in the tank with fresh water, and you get a final copper concentration of 3.9 ppb.
Does this happen in real products and not “ideal” products? I have no idea. But the statement by manufacturers that it contains all ions in natural ratios, including copper, should not be viewed as a concern that it is exacerbating a heavy metal problem.
The rise in salinity of these products over time can be very roughly calculated, though there are several reasons why this calculation is only an estimate. For every 1000 meq of alkalinity added in this fashion (and the matching amount of calcium) these products will deliver on the order of 60 grams of other ions to the tank. In a tank with a low calcification demand (defined later to be 18.3 thousand meq of alkalinity per year in a 100 gallon tank (0.4 dKH/day)) this effect will raise the salinity by 3 ppt per year (compared to a normal salinity of S ~35). In a high demand tank (defined later to be 219 thousand meq of alkalinity per year in a 100 gallon tank (4.4 dKH/day)), the salinity will rise by 35 ppt in a year, or approximately doubling the salinity. Consequently, the salinity should be monitored closely in using these types of additives, especially in a tank with high calcification rates.
Many people have begun to use dosing pumps to deliver these sorts of additives more uniformly across a day/night period with less work by the aquarist. Such pumps can be obtained starting under $100 for each part dosed this way. There is no need to dose the magnesium part this way, since very little is actually required and once a week is plenty often enough.
The costs of these systems vary a bit. The original B-ionic from ESV costs about $34 for 1 gallon of both parts (10,600 meq of alkalinity), or about $3.20 per thousand meq of alkalinity. It has a pH raising effect, similar to my DIY Recipe 1. The B-ionic Bicarbonate version is more expensive, and is necessarily more dilute than is the original because sodium bicarbonate is much less soluble than is sodium carbonate. If your tank pH gets too high using one of them (such as the original B-ionic), then it is reasonable to switch to one that has a smaller pH raising effect (like the bicarbonate B-ionic or my DIY Recipe 2 using baking soda).
The DIY recipes can be far less expensive, depending on what grade of ingredients you use. Buying ingredients from a place such as Bulk Reef Supply will cost roughly $10 per gallon (total cost of all parts, so 1 gallon calcium, 1 gallon alkalinity, and a few cups of magnesium additive), or about $1.40 per thousand meq of alkalinity.
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