Can't raise MG

[cdw79]

I do a 15% water change every week, pretty religiously. I dose two part, and I use a trident + Apex to keep CA and Alk in check. Over the past weeks and even months, my Mg very slowly as dropped from a pretty consistent 1250 or so, all the way down to 1165 (CA is 430, Alk is 8.62). I bought Kent Tech M Mg awhile back and started adding some here and there, but no change. I started adding quite a bit but nothing changed. I recalibrated my Trident a few weeks ago and there wasn't any correction. Any ideas? I know I'm now outside of recommended levels, so it needs to get fixed. Thanks for the help in advance!

 

[Randy Holmes-Farley Verified Community Expert]

I do a 15% water change every week, pretty religiously. I dose two part, and I use a trident + Apex to keep CA and Alk in check. Over the past weeks and even months, my Mg very slowly as dropped from a pretty consistent 1250 or so, all the way down to 1165 (CA is 430, Alk is 8.62). I bought Kent Tech M Mg awhile back and started adding some here and there, but no change. I started adding quite a bit but nothing changed. I recalibrated my Trident a few weeks ago and there wasn't any correction. Any ideas? I know I'm now outside of recommended levels, so it needs to get fixed. Thanks for the help in advance!

Use a calculator to add the recommended dose. It takes a ton to boost magnesium.

Reef Chemistry Calculator

Magnesium testing is also often inaccurate. Double check the test against new salt water.

 

[KQuillan]

Same thing was happening to me. I then tested the water I get from the LFS and it was extremely low (1070). I also ordered a new test kit.

 

[Garf]

Are you both confident your salinity is good?

 

[cdw79]

I checked the calibration on my refractometer just now and did another check on the display salinity- 1.024, as expected, so don't think I have any issue on that front!

 

[Garf]

I checked the calibration on my refractometer just now and did another check on the display salinity- 1.024, as expected, so don't think I have any issue on that front!

Ok, but if you were at 1.026, that would take your mag to the 1250ish, not done the maths though.

 

[KQuillan]

Yep, mine is at 1.025

 

[moz71]

I was having similar problem but dosing bright well mag and could not get it to rise then discovered I was dosing not enough. It takes a ton of dosing at the recommended mixing ratio to RODI water so had to completely saturate the solution to get it to rise. I am not sure on your product though

 

[nereefpat]

Ok, but if you were at 1.026, that would take your mag to the 1250ish, not done the maths though.

Right 1.024 is to 1165 as 1.026 is to 1262, so that would fix it.

As Randy said, it takes a lot of Mg supplement to raise it much.

I would also add that if you're doing weekly 15% water changes, there really isn't a need to supplement Mg.

 

[Garf]

Same thing was happening to me. I then tested the water I get from the LFS and it was extremely low (1070). I also ordered a new test kit.

Yep, mine is at 1.025

How are you checking salinity?

 

[KQuillan]

How are you checking salinity?

Hi Garf! I check salinity with a refractometer. However, let me respond with the latest. So I took a sample to the lfs along with my salifert test. Needless to say, they tested a bit differently than I do. I have been rushing through the testing shaking the vial the whole time while adding the drops. When slowing the shaking process down a bit, my test came back at 1350!

 

[cdw79]

Right 1.024 is to 1165 as 1.026 is to 1262, so that would fix it.

As Randy said, it takes a lot of Mg supplement to raise it much.

I would also add that if you're doing weekly 15% water changes, there really isn't a need to supplement Mg.

But in theory the WC should bring it back up to a higher MG level, but since it doesn't I'd conclude that just the water changes aren't going it, right?

 

[nereefpat]

But in theory the WC should bring it back up to a higher MG level, but since it doesn't I'd conclude that just the water changes aren't going it, right?

Not as much, if the saltwater is mixed up light. I would raise the salinity a couple points over the next few days by topping off with saltwater.

You could also mix up a batch of saltwater like you usually do, and see where the Mg level is.

Another thing to keep in mind is the standard error (+/-) of the test. It's going to be +/- 20 or 40 ppm of Mg.

On last thing to keep in mind is how much Mg is used in a reef. It's pretty slow. In fact, it's always in the order of about 1/10th the use of Ca. For example, if the tank goes through 2 dKH of alk per day (mine is about 1 dKH):
2 dKH per day = 14 dKH per week
That's about 100 ppm Calcium
About 10 ppm per week of Mg.
What is your daily Alk consumption?