Magnesium

iheartsps · Apr 23, 2016

Is it normal for my system to not use up magnesium?

Jej34 · Apr 23, 2016

I do 20% water changes bi-weekly and I never have to play with mag or calcium, just Alki @25 ml per day.

iheartsps · Apr 23, 2016

I read the optimal parameters thread after posting the question. I use a two part Kh and Ca. Good quality. So it's probably adding the Mag in as well which is why I don't see a drop in the Mag.

McMullen · Apr 23, 2016

I rarely dose mag in a Sps dom tank. Water change twice mo maintains.....mostly. Ball parking, but I probably add 30ml BRS mag one every three to four months.

jsker · Apr 23, 2016

In a newer system you are going to use up more Mg. it would help if we had all of your parameters.

Jej34 · Apr 23, 2016

iheartsps said:
I read the optimal parameters thread after posting the question. I use a two part Kh and Ca. Good quality. So it's probably adding the Mag in as well which is why I don't see a drop in the Mag.

You may want to research that further. If a two part system adds mag, it's news to me.

Randy Holmes-Farley · Apr 24, 2016

iheartsps said:
Is it normal for my system to not use up magnesium?

Yes. Magnesium depletion is so slow and some salt mixes have so much extra that many people do not need any (folks using a DIY two part are different because a lot is needed then to balance addition of other ions, such as sodium).

Randy Holmes-Farley · Apr 24, 2016

Jej34 said:
You may want to research that further. If a two part system adds mag, it's news to me.

Commercial Two Part systems do contain magneiisum and should add enough to keep it stable (unless water changes are lowering it).

Same for my DIY recipe if you use all three parts (BRS, for example, sells the ingredients).

Here's the recipe and the rationale: for the amount of magnesium in it

An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System by Randy Holmes-Farley - Reefkeeping.com

from it:

Residual Ions from the Calcium and Alkalinity Parts

Adding 1 gallon of each of these additives will result in a residue of ions remaining after calcification. These are mostly sodium and chloride, and the amounts of those two added are equal in numbers (i.e., moles), but slightly different in weight-based concentrations such as ppm because they do not weigh the same.

After adding 594 grams of baking soda (1 gallon of Recipe #1), we will have added 163 grams of sodium. In natural seawater, magnesium is present at about 12.0% of the sodium concentration (by weight). In order to match the magnesium additions to the sodium additions to leave them in a natural ratio, we need to add 12% of 163 grams, or 19.5 grams, of magnesium for every gallon of the two-part additive that we add.

Additionally, we may want to account for magnesium that is actually incorporated into the coral skeletons. For this calculation, I have assumed that the amount of magnesium incorporated is about 6.5% of the calcium level (by weight), or about 2.5% of the skeleton by weight. In the course of adding this gallon of both parts of the two part supplement, we added 141 grams of calcium, so we need to add 0.065 x 141 = 9 grams of magnesium to account for this deposition.

The magnesium parts of the recipe are designed to add enough magnesium so that it is not depleted by either of the two means described above. Because the magnesium supplement (either version) is 47,000 mg/L in magnesium, we need to add (9 +19.5) grams/47 g/L = 610 ml of the magnesium solution for each gallon of the other parts of Recipe #1.