Magnesium Testing - Which One to Believe?

dbl · Apr 26, 2015

I recently purchased a Red Sea Pro Test Kit to see how it compares to my current testing methods. The Calcium is nearly identical to the Salifert and the Alkalinity is very close to the Hanna Checker. My problem comes with the Magnesium. Tonight, Salifert read 1380 and the Red Sea 1460. I’ve been experimenting for more than a month now and Red Sea always reads about 60-80 ppm higher. I’m finally getting around to asking for advice, guidance, or for someone to tell me I’m an idiot and over thinking all this!

So, which should I believe? I’m trying to dial in the dosing system (which I recently started) and was using the Salifert numbers as my starting points. So I’ve either overshot Magnesium by a little or a lot. Maybe in the overall scheme of things, they are close enough given these are just hobby grade test kits.

Below are some system details (hopefully I’ve covered all eventual questions) and I will now shut up and listen!

Nearly one year old 120g with 55g sump, skimmer, ATO, carbon and GFO reactor, 4 AI Vegas, 20% weekly water change, using Red Sea Coral Pro Salt Mix. Dosing Cal/Alk/Mag, or at least trying to dial them in correctly. Other parameters this evening:

Salinity: 1.026, Milwaukee Refractometer
Alkalinity: 7.504, Hanna Checker
Calcium: 415, Salifert/Red Sea
Magnesium: 1380 Salifert/1460 Red Sea
pH: 8.2, Pinpoint Monitor
Nitrites/Nitrates: 0, API
Phosphate: 0, Hanna Checker
Temp: 80.1, Probe (I live in SW Florida…it’s hot here…always)
Life: 12 fish + CUC
Corals: Mixed Reef – Softies, LPS, SPS

Thanks in advance for any light you can shed on this.

NanaReefer · Apr 26, 2015

Both numbers are acceptable. Pick one and go by that [emoji13] Personally I'd stop dosing MAG.

akillez · Apr 27, 2015

I have exactly the same issue. Now I just use the red sea kit though slightly more labour intensive. I knock off about 25-30ppm which seems to be the middle ground between the Salifert and red sea. This suits me as I'm looking for a trend rather than a number.

Randy Holmes-Farley · Apr 27, 2015

I don't know which (if either is accurate), but I don't think the difference between 1380 ppm and 1460 ppm is very important for magnesium. It is only about 5% of the reading, and magnesium is generally fine from about 1250 to 1500 ppm.

FWIW, a similar magnitude of error in alkalinity would only be from 7. to 7.4 dKH.

dbl · Apr 27, 2015

I had turned off the Mag pump in anticipation of responses. So I will check it mid-week and determine the consumption – again.

Randy…I don’t ask a lot of questions because most can be answered through reading, which I do a lot of. The more I’ve looked in to this topic, and based on your response above, the more I’m starting to believe that stability/consistency is more important (or at least as important) as the actual testing result. There are hundreds of threads regarding which test kits are better, and thousands of answers!!!

So I’m thinking it’s best to find a test kit you like, get the parameter within the desired range, and do what you need to do to keep it there. Maybe it really is that simple and I answered my own original question – I’m an idiot and over thinking all this.

Thanks to all for your help...I appreciate your taking the time. Happy Reefing.

NanaReefer · Apr 27, 2015

dbl said:
I had turned off the Mag pump in anticipation of responses. So I will check it mid-week and determine the consumption â€“ again.

Randyâ€¦I donâ€™t ask a lot of questions because most can be answered through reading, which I do a lot of. The more Iâ€™ve looked in to this topic, and based on your response above, the more Iâ€™m starting to believe that stability/consistency is more important (or at least as important) as the actual testing result. There are hundreds of threads regarding which test kits are better, and thousands of answers!!!

So Iâ€™m thinking itâ€™s best to find a test kit you like, get the parameter within the desired range, and do what you need to do to keep it there. Maybe it really is that simple and I answered my own original question â€“ Iâ€™m an idiot and over thinking all this.

Thanks to all for your help...I appreciate your taking the time. Happy Reefing.