Low Mg

[thunderdood]

Over the weekend I found my Mg was reading around 800. I thought my Red Sea test kit was off so I went to my LFS and they did not have a Mg test kit to test my water so they gave me a bag of their water that they said was 1390. I tested the LFS water and with my test kit their Mg was around 1360. I've been dosing Mg throughout the weekend and brought it up to 900 last night which was spot on to the amount I dosed with the Red Sea Mg supplement for the last 2 days.

As of late last night here are my readings:
Mg - 900ppm
Alk - 7.5dKH
Ca - 440-460ppm
Salinity - 1.025

My tank is a 20g with 10g sump. Total water volume around 25-27g. I forgot which salt mix I use but I also did a 20% water change the day before I tested which confused me on my readings. I had a feeling something was off since a couple SPS frags haven't had the normal polyp extension and the Coraline stopped trying to take over all the glass.
Is there anything else I should be doing other than dosing to get the Mg up?

 

[Randy Holmes-Farley]

Test error, somehow, is by far the most likely explanation. It does not just drop that low.

800 ppm magnesium is not ever anything else except a faulty salt mix. Did the previous water come from the LFS?

 

[thunderdood]

I only buy fresh water from the LFS and use my bucket of Reef Crystals to do water changes. The LFS gave me a bag of their salt water so I could have a second reference to test with. I never actually tested the salt mix levels before doing a water change. I will do that when I get home.

 

[Randy Holmes-Farley]

The only explanation other than test error is that the water you got from the LFS had very little magnesium in it.

 

[rkpetersen]

From your description of what you've tried, it sounds like your Red Sea test is working ok. But I'd still check your result with a Salifert test kit. Something odd going on there, with such an improbably low number. I wonder whether there could there be something in your circulating water that interferes with the test. That would allow the same test kit to give accurate numbers when checking the lfs water or freshly mixed saltwater.

 

[thunderdood]

I tested the tank water again after dosing last night and the Mg has increased to 1000 ppm. Today is day 3 and from what I've read to not go over 100ppm/day. Mg dosing seems to be spot on...

 

[nkyreef]

What's your salinity? Is there a chance not enough salt was added when doing a water change?

 

[WWIII]

Sounds like your dosing and testing is lining up. I'm wondering how the lfs knew their magnesium level if they didn't have a test kit?

 

[thunderdood]

What's your salinity? Is there a chance not enough salt was added when doing a water change?

Salinity was 1.025 after I did the water change. I did the usual 1g to half cup of salt ratio I think it is.

Sounds like your dosing and testing is lining up. I'm wondering how the lfs knew their magnesium level if they didn't have a test kit?

Great question but I'm not sure. I assume they get it from somewhere else. But they said Mg is 1390 and I tested their water that came out to ~1360 which is about a 2% difference.

 

[nkyreef]

That's the ratio I mix reef crystals. Have you tested the mag in a gallon of newly mixed saltwater? I've measured 1240 mag in new water before.

 

[Randy Holmes-Farley]

The fact that it is rising doesn't really say anything much about the accuracy.

 

[thunderdood]

So I checked a 1g mix and Mg ~760ppm, Ca ~360ppm, ALK 6.5dKH.

 

[rkpetersen]

Something very wrong there, I think you've found the problem. Perhaps your salt wasn't properly mixed or had separated in the bag. Is this result with salt from the top or bottom of a bag? Do you mix each new container of salt so that it's uniform, before using it to make water?

Also, check calibration of your refractometer, just in case.

 

[thunderdood]

I bought the salt bucket mix of Reef Crystals and I'm a little more than half way done with the bucket. I haven't actually mixed the salt around in the bucket. I've just been scooping the salt and putting it into another bucket to mix with water.

 

[Randy Holmes-Farley]

So I checked a 1g mix and Mg ~760ppm, Ca ~360ppm, ALK 6.5dKH.

Salt mix?

Generally, sampling a small volume is prone to issues due to the mix not being homogeneous. The different size and density salt grains can separate during shipping.

 

[thunderdood]

Salt mix?

Generally, sampling a small volume is prone to issues due to the mix not being homogeneous. The different size and density salt grains can separate during shipping.

Reef Crystals.

I'll try mixing up the Reef Crystals salt before using it for a WC and do another test.

 

[Randy Holmes-Farley]

OK, sounds good.

 

[reefruss]

Test error, somehow, is by far the most likely explanation. It does not just drop that low.

How low can it be expected to drop? I just tested my water yesterday and I got a Mg reading of 730 using a Salifert test. I've been dosing two part but not Mg, yet. My calcium reads 570 and Alk is 9.0. Would you expect the salt mix or the test kit being at fault?

 

[Randy Holmes-Farley]

How low can it be expected to drop? I just tested my water yesterday and I got a Mg reading of 730 using a Salifert test. I've been dosing two part but not Mg, yet. My calcium reads 570 and Alk is 9.0. Would you expect the salt mix or the test kit being at fault?

Magnesium drops 0-2 ppm per day when dosing calcium and alk with no magnesium and no water changes, and will drop less as the magnesium drops. I don't recall ever seeing magnesium below 1000 ppm without it being error.

 

[Randy Holmes-Farley]

Are you sure you are not reading the syringe backwards?