Reef Chemistry Question of the Day #8

Randy Holmes-Farley

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Reef Chemistry Question of the Day #8


Assume that you have a coral reef aquarium that has almost perfect water chemistry. It matches natural seawater in every possible way, except you've discovered that your magnesium test kit was off, and that magnesium is actually 100 ppm too low.

You know from years of aquarium husbandry that there are a variety of ways to boost magnesium, and that some of those will mess with your otherwise perfect water chemistry more than others.

Rank order the following methods from the one that least changes the water chemistry to the one that changes it the most. The changes should be compared and ranked on the basis of how you think the aquarium organisms will be impacted by the changes. For example, you might believe that a doubling of calcium would have more impact than a doubling of strontium, so if you thought those would happen, you'd rank the calcium change as a bigger disruption of your perfect chemistry.

The methods are:

A. Adding magnesium chloride hexahydrate to boost magnesium by 100 ppm.
B. Adding magnesium sulfate heptahydrate to boost magnesium by 100 ppm.
C. Adding 10 parts by volume of magnesium chloride hexahydrate and 1 part by volume magnesium sulfate heptahydrate to boost magnesium by 100 ppm.
D. Adding dolomite to your calcium carbonate/carbon dioxide reactor and running it until magnesium is boosted by 100 ppm

The answer should look something like:

W<X<Y<Z, with Z being the most disruptive to water chemistry

Good luck!
 
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Galvinized

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At a guess I'd say it's A-C-B-D

It is tricky because of the sulfate, it's possible C & B could be in the wrong places but the others I'm comfortable with.
 

Galvinized

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At a guess I'd say it's A-C-B-D

It is tricky because of the sulfate. I did think that C & B could swap but if the levels are at nsw then I'll stick with the order I've posted.
 
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Randy Holmes-Farley

Randy Holmes-Farley

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Well, the winner is Kyuss414! Excellent!!! This was a hard one.


Since this is his (or her) first and only post on R2R, maybe he's a ringer that signed up just to answer the question. :D

C<A<B<D

Here's my rational for that answer.

D is the worst method, in my opinion, because boosting magnesium by 100 ppm will boost alkalinity by about 8.5 dKH. So if the tank started at 7 dKH, it would end with magnesium OK, but alkalinity at 15.5 dKH. The reason this happens is because it is effectively dissolving magnesium carbonate to release both magnesium and the carbonate.

C is the best method because, assuming the chemicals are pure, it adds the magnesium without changing the relative ratio of chloride to sulfate in the water.

A is the next best because there is far more chloride than sulfate in seawater. So boosting the chloride alone is a fairly minor tweak.

B is the third best choice. Since there is much less sulfate than chloride, adding all the sulfate will drive it up a considerable amount, causing a boost in the sulfate to chloride ratio.

FWIW, the distinction between B and A in preference is fairly minor, but I'd always pick B if it is a choice.
 
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Galvinized

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Ahhha, I see. So would it be safe to say that the balance between the sulfate & chlorine is more important than the value? (With regards to elevated sulfate)

Sorry to take this thread off topic but I do have one question: what's your thoughts on Balling Classic without the sulphate? Good/bad? Long term/short term?
 

Kyuss414

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Well, the winner is Kyuss414! Excellent!!! This was a hard one.


Since this is his (or her) first and only post on R2R, maybe he's a ringer that signed up just to answer the question. :D

C<A<B<D

Here's my rational for that answer.

D is the worst method, in my opinion, because boosting magnesium by 100 ppm will boost alkalinity by about 8.5 dKH. So if the tank started at 7 dKH, it would end with magnesium OK, but alkalinity at 15.5 dKH. The reason this happens is because it is effectively dissolving magnesium carbonate to release both magnesium and the carbonate.

C is the best method because, assuming the chemicals are pure, it adds the magnesium without changing the relative ratio of chloride to sulfate in the water.

B is the next best because there is far more chloride than sulfate in seawater. So boosting the chloride alone is a fairly minor tweak.

A is the third best choice. Since there is much less sulfate than chloride, adding all the sulfate will drive it up a considerable amount, causing a boost in the sulfate to chloride ratio.

FWIW, the distinction between B and A in preference is fairly minor, but I'd always pick B if it is a choice.

haha, I just found this place and signed up a few days ago and seen this, then decided to take a shot at it today since there still weren't too many replies.

I used the same rational, and was most questionable about the order of A&B (BTW you have the A & B rationals switched ;) ) but again thought the same - there's more chloride than sulfate so it'd be a smaller change.

That said, I learned this stuff from reading (and re-reading, til it started to make some sense to me lol) your articles Randy, so hats off to you! :insertbeersmileyhere:
 
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