As I often say, there's no perfect method to supplement calcium and alkalinity. All have pros and cons of various sorts.
The imbalance with calcium to alk is one of the cons of kalkwasser.
I resolved it by just using a high alk/lower calcium mix (normal IO) and calcium balanced out at 472 ppm long term using 1% daily water changes.
The boost to sodium from an occasional correction is, of course, not nearly as large as using just sodium carbonate and calcium chloride for alk and calcium.
let's look...
Suppose you add 2 dKH of alk every day using kalkwasser. That means adding 14.3 ppm of calcium each day. A pretty high demand scenario.
Take an extreme case where you have the most magnesium-consuming organism we typically keep: coralline algae.
This article details the nature of magnesium in seawater, how it is added, measured, and removed from marine aquaria, and how it impacts the maintenance of calcium and alkalinity.
reefs.com
Pure calcium carbonate is 40% calcium (and 60% carbonate) by weight.
Let's assume coralline is as reported in the literature for Corallina pilulifera to be 4.4% magnesium by weight in its calcium carbonate. That 4.4% magnesium displaces calcium to the extent of 7.2% of the weight of the calcium carbonate (because calcium weighs more than magnesium, more of the weight % of calcium is removed for the given weight percent of magnesium incorporated).
Thus, for each carbonate deposited, we are using less calcium. The magnitude is about 40-7.2/40 = 82% of what we would use for pure calcium carbonate.
In our original scenario (pure calcium carbonate) we assumed:
2 dKH per day
14.3 ppm of calcium each day
But in this coralline consumption, we are consuming about:
2 dKH per day
14.3 x 0.82 = 11.7 ppm calcium per day
1.6 ppm magnesium
Now we just need to calculate how much sodium carbonate (or bicarbonate or hydroxide, all give identical results) is needed to offset this difference between the calcium added and the calcium consumed.
Difference is 14.3 -11.7 ppm per day = 2.6 ppm per day
Using our consumption ratio of 2 dKH per 11.7 ppm of calcium, we now know tht we need to add extra alk to the tune of 2 dKH *(2.6 ppm/11.7 ppm) = 0.44 dKH per day.
How much is that and what does it do to sodium?
0.44 dKH is 0.16 meq/L or 0.16 mM sodium bicarbonate.
0.16 mM sodium bicarbonate contains 3.7 mg/L sodium (since sodium weighs 23 mg/mmole).
Thus, we are adding 3.7 ppm of sodium each day.
In the course of a month we are adding 115 ppm. The normal background of sodium is about 10,500 ppm, for a 1.1% rise in sodium.
If you maintain salinity, that rise gets corrected to a 0.33% rise in sodium and a 0.66% drop in everything else (since sodium is about 1/3 of the mass of ions in seawater).
Is this significant in a month or a year?
Month? Definitely no. A year? Maybe, but I doubt it.
For comparison to the variability in sodium caused by different salt mix and different processes in reef tanks that folks think are good, take a lot at a bunch of random ICP values for sodium.
I'll do it now, but your results will vary.
Here's what I find for the most recent 10 ICP values I find by searching this forum:
10,468 ppm
10,495
13,486 (high salinity issue)
11,654
11,027........10,945......(two ICP companies, same tank)
9653
10,348
10,210
11,154
10,210
10,100
As we can see, there's lots of variability in sodium between tanks, and there no evidence that sodium a little high or low is a concern.
