Reef Crystals Mixing Low in Magnesium

[youclowntoomuch]

Bought a new bucket of reef crystals that is mixing low in Magnesium(1200). Made sure to tumble it well before using. Not sure if it is my magnesium test kit from salifert thats erroneous, or if its a bad batch of salt. What do yall think?

 

[KrisReef]

I would test my main tank to see if it’s also “low” and for Mg I probably would add some Mg into the new batch just to make sure it wasn’t low. I don’t think elevated Mg is a bad thing so I would error towards the high side, but yeah the test kits are often not all that reliable for Mg measurements.

 

[youclowntoomuch]

I would test my main tank to see if it’s also “low” and for Mg I probably would add some Mg into the new batch just to make sure it wasn’t low. I don’t think elevated Mg is a bad thing so I would error towards the high side, but yeah the test kits are often not all that reliable for Mg measurements.

I just had a bad experience with a batch of salt wreaking havoc on my tank params. Not trying to repeat that lol

 

[Euphylliaphyle]

I just tested some freshly-mixed Reef Crystals in RO/DI using Salifert titration:

Ca: 455
Alk: 7.5
Mn: 1,160

Lot 19235 22:18

Mixed to 35 ppt using Tropic Marin glass bulb hydrometer and temperature-corrected.

 

[Randy Holmes-Farley Verified Community Expert]

I don’t have confidence in the accuracy of home magnesium testing.

 

[Euphylliaphyle]

I don’t have confidence in the accuracy of home magnesium testing.

I understand that, but awhile ago I did 3 consecutive tests with my kit and got, surprisingly, exactly the same result 3 times from the same sampling. I don’t know how accurate it is because I don't have a standard, but the repeatability is pretty good. I might whip up a standard using your Recipe and try that again. (Let's put a pin in that for later).
I have been testing my tank and it has been coming up consistently around 1,300 to 1,350. So to me, there is a significant difference between what I have had (1,330-ish) and what I just mixed (1,160) that goes beyond demonstrated measurement variability.

 

[Randy Holmes-Farley Verified Community Expert]

I understand that, but awhile ago I did 3 consecutive tests with my kit and got, surprisingly, exactly the same result 3 times from the same sampling. I don’t know how accurate it is because I don't have a standard, but the repeatability is pretty good. I might whip up a standard using your Recipe and try that again. (Let's put a pin in that for later).
I have been testing my tank and it has been coming up consistently around 1,300 to 1,350. So to me, there is a significant difference between what I have had (1,330-ish) and what I just mixed (1,160) that goes beyond demonstrated measurement variability.

What kit are you using?

 

[Euphylliaphyle]

What kit are you using?

Salifert

 

[rtparty]

During my big salt test I had 2 Salifert kits read super close to ICP for a little while and then just totally stop working out of nowhere. They didn’t expire, weren’t left open, nothing. Just all of the sudden they read every salt over 1500ppm of magnesium no matter how I tested. ICP would come back with the more expected 1300-1350ppm as an example.

The real conundrum here is do we trust the magnesium test kit or the low quality salt? Or are they both “wrong” and the issue is compounded?

 

[rtparty]

Double post

 

[Euphylliaphyle]

@rtparty , I have another tank test day coming up on Friday. I'll see if my kit has gone haywire then. Thanks for the info.

 

[Euphylliaphyle]

@Randy Holmes-Farley , if I add 108.8 ml of your Recipe 1, Part 3 for Mg to 1 gallon RO/DI water (sorry for the mixed systems), will that suffice for a 1,350 ppm Mg standard, or will the small volume RO/DI impact the calculations too significantly to be useful? Am I correct in thinking that no molar computations are necessary? For the Part 3, I weighed the salt on a precision balance and measured the water with a semi-precision graduated cylinder. I can't account for any moisture entrained by the Epsom salt before use.

 

[Randy Holmes-Farley Verified Community Expert]

One of the tricky things for magnesium and calcium testing is to not detect any of the other and detect all of the one you want. That is one reason a seawater matrix makes it much harder to properly detect magnesium than that sort of standard would evaluate.

 

[Euphylliaphyle]

One of the tricky things for magnesium and calcium testing is to not detect any of the other and detect all of the one you want. That is one reason a seawater matrix makes it much harder to properly detect magnesium than that sort of standard would evaluate.

So even a perfect standard, or even the very act of Mg testing would be futile due to confounding?

 

[Randy Holmes-Farley Verified Community Expert]

So even a perfect standard, or even the very act of Mg testing would be futile due to confounding?

A seawater sample with known magnesium is perfectly suitable. Either natural ocean water with known salinity, or even a tank water sample that had a quality icp test.

 

[Euphylliaphyle]

A seawater sample with known magnesium is perfectly suitable. Either natural ocean water with known salinity, or even a tank water sample that had a quality icp test.

Well, I sent in an ICP-MS (Oceamo) on 3/30/26 and did my own testing within a half hour of ICP sampling, just for this purpose, so we'll see! I was hoping for something I could use repeatedly over time to stay zeroed, but I guess that's impractical. Thanks.

 

[Randy Holmes-Farley Verified Community Expert]

Well, I sent in an ICP-MS (Oceamo) on 3/30/26 and did my own testing within a half hour of ICP sampling, just for this purpose, so we'll see! I was hoping for something I could use repeatedly over time to stay zeroed, but I guess that's impractical. Thanks.

That will be perfect. Did you save any for future use? Magnesium in seawater is perfectly stable.

 

[Euphylliaphyle]

That will be perfect. Did you save any for future use? Magnesium in seawater is perfectly stable.

No, I didn't have the foresight to do that. I live an hour from the Atlantic Ocean, so I can grab a gallon next time I'm at the beach to keep on hand.

 

[Randy Holmes-Farley Verified Community Expert]

No, I didn't have the foresight to do that. I live an hour from the Atlantic Ocean, so I can grab a gallon next time I'm at the beach to keep on hand.

You would need to be sure it is away from river inputs, and your salinity measurement needs to be very accurate. The difference between a sg of 1.025 and 1.026 is about 49 ppm.

 

[Euphylliaphyle]

You would need to be sure it is away from river inputs, and your salinity measurement needs to be very accurate. The difference between a sg of 1.025 and 1.026 is about 49 ppm.

Makes sense, but how far away is "away?"