Confused yet again by Magnisium

[ShaggyRS6]

So! I did my weekly water change, made sure my ALK was the same level as the tank, i dosed some ALK into my water change water to bring it up to 8.2. Checked the magnesium level of the mix, that was at 1350. Great I thought, that will bring my mag up a little from 1275 and I can let it just drop again over the week and rinse and repeat. WRONG!

The Trident tested about 4 hours after the water change and it dropped to 1266! Confused, dont know what to do!

 

[watdachuck]

I would recommend these articles by RHF as a reference. The first is the interaction between Calcium, Alkalinity and Magnesium. The second is similar with a deeper focus on Magnesium.

http://reefkeeping.com/issues/2006-06/rhf/index.php

Aquarium Chemistry: Magnesium In Reef Aquaria

This article details the nature of magnesium in seawater, how it is added, measured, and removed from marine aquaria, and how it impacts the maintenance of calcium and alkalinity.

www.advancedaquarist.com

 

[nereefpat]

1275 to 1266 is a REALLY small change, and likely just within the error of the test. Call it the same value.

How big of a water change did you do?

 

[ShaggyRS6]

10 Gallons. Calcium went up to 490 from 470, ALK remained the same as did salinity , well within a decimal point SG went from 36.7 to 36.6

 

[nereefpat]

10 Gallons

What is the volume of the tank?

 

[ShaggyRS6]

What is the volume of the tank?

100g

 

[beaslbob]

I agree that 1350 to 1250 or so is just experimental error on our hobbist test kits.

 

[ShaggyRS6]

It’s taken on a trident ! Backed with Salifert.

 

[nereefpat]

10 gallons x 1350ppm = 13500
90 gallons x 1275 = 114,750
13,500 + 114,750 = 128,250
128,250/100 = 1282.5 ppm ...theoretically doing a 10 % water change with 1350 ppm saltwater.

1282, 1275, 1266 is all about the same. Average is 1274 with a standard deviation of about 8 ppm. I would call it the same.

 

[ShaggyRS6]

Ok none of that made sense and I say that with the greatest respect. In my simple little brain, if I am putting water in, with a mag level Which is great than is in there, I should see an increase in mag and not a decrease ?

Unless I am missing how the mag works which is highly likely!

 

[nereefpat]

We, with our test kits, are not able to tell the difference between a water sample with 1275 ppm Mg and a water sample with 1266 ppm Mg. That means we have to assume the value is the same. Between standard error and the resolution of the test kit, we can't tell the difference between the two samples.

Your 10% water change probably increased the Mg by about 7ppm. Again, your test kit can't tell the difference.

Does that make sense?

 

[ShaggyRS6]

Nope. Should it not have gone up or at very least stayed the same on the Neptune Trident?

 

[nereefpat]

According to Neptune's website, the Magnesium reading is + or - 15 ppm.

That means if the Neptune Trident gives you a reading of 1275 ppm, the actual value of Mg in the tank is 1275 +/- 15 ppm. In other words, the Mg in the tank is somewhere between 1260 and 1290 ppm.

 

[ShaggyRS6]

Ok, now it makes sense! I should have read the that detail on the Neptune site. I’m still sceptical though I appreciate your patience with me in this one

 

[nereefpat]

I'm glad. Sorry I couldn't communicate what I was thinking clearer.

I wouldn't make a very good Statistics teacher, apparently.

 

[Randy Holmes-Farley Verified Community Expert]

Ok none of that made sense and I say that with the greatest respect. In my simple little brain, if I am putting water in, with a mag level Which is great than is in there, I should see an increase in mag and not a decrease ?

Unless I am missing how the mag works which is highly likely!

here’s the simple version: the result you got reflects imperfect testing, not an actual magnesium change.

 

[ShaggyRS6]

here’s the simple version: the result you got reflects imperfect testing, not an actual magnesium change.

Hey!!! Leave my trident alone

 

[Randy Holmes-Farley Verified Community Expert]

lol