Odd Magnesium swing

[Pittsburgh]

I just had a very odd reading of magnesium the other night...

I do BRS's two part dosing. Test for Cal and Kh once a week and Mag about once a month. So it's been about 3 weeks since I tested for Mag, usually the number would be spot on just around 1350ppm so I never worried. The other night decided to do the test with Red Sea pro and it came back at 750ppm :thumbdown

Cal tested low as well (~390) and Kh at around 10.5. So I tested Mag again right away, same result ~ 750ppm. I figured something's going on but I still didn't buy the result. Dumped about 7oz of BRS's Mag solution just in case and decide to wait out until the next night.

So last night came back and tested all parameters again. Mag is now at ~1400ppm, Cal at 420ppm, Kh at 9.5 (this is more of a norm for my tank). So given that it would take like 1.5 gallons of Mag solution to bring it from 750 to 1400 in my 95Gal tank and I certainly didn't use that much, the main suspect is the test. But then again, I tested and retested it that night, and the following night I was using the same test kit...

Temperature is solid at 78F. Salinity is 1.025 rock solid due to ATO. The night before the bad test I did do my biweekly 15% WC. I use Reef crystals.

What gives?

[disc1]

Most likely made a mistake testing. It happens to the best of us.

[downbeach]

The Red Sea Mg test is a little bit laborious, since you have to mix each drop of the reagent A for 15 seconds between each drop. It's a PITA, but if you don't do it this way, you will get an erroneous result.

Here's a video:
http://www.youtube.com/watch?v=MofaqM-I6uM

[Pittsburgh]

A screwed up test was also my first guess... but I did several tests that night and came up with generally the same result every time.

I did everything just like the video says. I do agree that shaking the solution for 15 sec between each drop is really annoying...

From chemistry stand point though, I wonder what may have affected the reading assuming that the test was performed properly. I've heard that if the salinity is off, Mg will read differently. Anything else?

[bertoni]

The magnesium results sound like testing problems to me, too.