Where did the Mg2+ go?

[cb684]

I have two systems one of approximately 400 gallons and one of approximately 200 gallons. Last Saturday I measured [Mg2+] in both of them:
400 gal: 1185 ppm
200 gal: 1050 ppm

I added about a gallon of a Mg2+ solution during 3 days in both systems. Today I got the following [Mg2+]:
400 gal: 1170 ppm
200 gal 1200 ppm
I used Salifert to test and checked twice.

Background:
The 400 gal system is about 7 months-old and has a lot of coraline, many small sps colonies (80+), few LPS (10 - 15), and few larger sps colonies (5-6).
The 200 gal system is 1,5 months-old and have little coraline and no corals at this time.

So, by my calculations the solution is approximately 8,000 ppm w/v, since it 1 gallon of solution raised 150 ppm in 1:200 dilution.
How could that disappear in the 400 gallon system?
What would be your next step to figure this out?
Thank you.
CB

 

[JimWelsh]

Were the "before" numbers checked twice also? What was the salinity, both before and after, of each of these two tanks? Do you follow the Salifert instructions closely, e.g., do you swirl for the full 30 seconds after adding the Mg-1 reagent? Just checking on certain things that might be relevant.

 

[biom]

Can you give a little bit more info about solution you use? Calculation of concentration based on tests before and after dosing is not always relevant.

 

[Randy Holmes-Farley Verified Community Expert]

Almost certainly the explanation is testing error, unless we do not understand what you did.

You added 1 gallon to each, or 1 gallon split between the two?

If this magnesium solution is something that follows my DIY recipe (such as BRS recipes), then if you added 1 full gallon to 400 gallons of water volume, the magnesium would rise by ~117 ppm.

There's no mechanism for significant uptake in a reef system aside from precipitation into calcium carbonate (with 0-4% magnesium in it depending on the way it was deposited by corals, coralline algae, or precipitated abiotically).

So for a tank to consume 100 ppm of magnesium would require the consumption of about 400 ppm of calcium and about 56 dKH of alkalinity.

Since that certainly didn't happen in 3 days, the only explanation is testing error.

FWIW, such errors are quite common.

 

[bct15]

How old is your test kit? I bet your test kit is bad. Magnesium test kits don't last very long, especially salifert kits.

Have your water tested somewhere else with a different kit as compare the results.

 

[cb684]

Thank you. I will get a new kit. I was thrown out a little by the test showing increase in one tank but not in the other, and that is why I was looking for other possible causes (precipitation, consumption, or test interferences).
During this time there was salinity fluctuation mainly in the new system where I increased the salinity from 1.023 to 1.026.
Unfortunately I used Seachem Mg, and lost my notes on its concentration.
Before adding more, I will test the solution comparing readings for my tank's water and then it after spiking it with the Mg2+ solution. I will do in triplicate so to access how much variability I am getting on this test.
Again, thanks a lot for the inputs.
CB

 

[Randy Holmes-Farley Verified Community Expert]

You're welcome.

FWIW, increasing specific gravity from 1.023 to 1.026 will boost magnesium by 13%, so, say, 1200 ppm to 1356 ppm.

 

[loglan]

If you are still using red sea blue bucket the mag in new salt water is a little lower to my liking. At 1.025 I get roughly a reading of 1260. I try to keep my mag around 1400 so I add mag to the new saltwater mix. Not sure where you lIke to keep yours.