Test Kits or Salt mix, which one do I have to trust?

[otimer]

Hi,
I recently reset my 100G reef tank and I use Tropic Marin Pro Reef salt mix (200G bucket).
I measured the calcium and magnesium of my tank water and sample mixing with RO/DI.
Also I tried a dry-mix of the salt.

The tank water measurements (repeated) : Calcium 400, magnesium 1100 ppm initialy.
The calcium and magnesium are going down gradually, now (after three weeks) 350 and 950.
There are no live stocks in the tank. 56Kg cured live rock and currently green hair algae only.
I think this make sense except the initial measurement.

TM says on their home page (US and Germany)
Alkalinity : 7~8 dKH
Calcium : 430~450 ppm
Magnesium : 1300~1350 ppm
I measured sample mix of 500mL of the salt with RO/DI after about 24 hours
Alkalinity : 130~138 ppm (7.28~7.84)
Salinity 34~35 ppt
Temp : +/-26~28’C
Calcium 390~410
Magnesium 950~1100

I measured Oceanic salt mix and the results are as follows.
Alkalinity : 165~175 ppm (9.24~9.53 dKH)
Salinity : 34~35 ppt
Temp : +/-26~28’C
Calcium +/-420
Magnesium +/-1200
I have read many postings that this salt has very high calcium level.

Instruments & Test Kits
Alkalinity : Hanna Checker HI755
Salinity : Red Sea Refractometer
Calcium : Red Sea Calcium Pro test kit (two test kits 08/2017 and 05/2018)
Magnesium : Salifert Magnesium test kit (two test kits similar validity)

The instruments are brand new. Different test kits show about 20 ppm difference for calcium, and 100~150 ppm difference for magnesium.

Which one is suspicious? All four test kits? Salt? Temperature? Poor measuring skill? (My background is industrial control & instrumentation. Hi~)
I cannot decide references for the future parameter management of my tank.
Please help!!
Thanks for helps in advance.

[Ron Reefman]

Stop worrying about a small difference in readings. The exact number is far less important than maintaining a stable environment. So you may be off by 20 or 40 ppm on calcium, if you always keep it at 400 to 450 you'll be fine.

Salt specs are just averages and every bucket can vary. Test kits are not fine instruments and you can prove that by trying several different brands on the same sample of water and get several different results. And then there is the skill of the used? It really isn't about skill, it's about doing it the same way every time.

[ReefsandGeeks]

+1, most people see variation between the advertised numbers vs tested numbers of their saltmix. It's more of a guideline than a hard number. besides, once you put it in your tank, the chemistry will start changine as elements are used up and others are added through feeding...etc.

I'd go with using your test kit as the reference point for matching up perameters because this is the only way you can test the water that is in your tank. If your kit is 20ppm lower than the true number, than it will show that on both new water and tank water (assuming perfect measuring of each). No need to stress over small variations though, as mentioned by Ron.

[jurgenph]

is it possible that you're not testing properly?

your Ca and Mg readings for TMPR and Oceanic are rather low.
Oceanic especially is ridiculously high in both Ca and Mg.

the Alk test results seem normal.

J.

[2_zoa]

Originally Posted by jurgenph

is it possible that you're not testing properly?

your Ca and Mg readings for TMPR and Oceanic are rather low.
Oceanic especially is ridiculously high in both Ca and Mg.

the Alk test results seem normal.

J.

How does one test incorrectly?

In my experiences I've done some tests in different ways. All be it, omitting the time references and gotten the same results. When I say time......I mean the addition of other reagents to the test. Not result time frame.
That's across API, Elos, Salifert.

To me......to test incorrectly. Means a different number of drops then the test requires. OR......an incorrect amount of reagent (read sea) to get the reading. I'd suspect that to be an individual going to fast.

[Kevin Guthrie]

For real fun look at the thermometers they have in the LFS. Why don't they all read the same temperature? They are in the same air...

[jurgenph]

Originally Posted by 2_zoa

How does one test incorrectly?.

When there is a human involved there are many things that can go wrong

I'm not saying he is... But the Hanna readings he gets are in line with what is expected.


J.

[Ron Reefman]

Originally Posted by 2_zoa

How does one test incorrectly?

In my experiences I've done some tests in different ways. All be it, omitting the time references and gotten the same results. When I say time......I mean the addition of other reagents to the test. Not result time frame.
That's across API, Elos, Salifert.

To me......to test incorrectly. Means a different number of drops then the test requires. OR......an incorrect amount of reagent (read sea) to get the reading. I'd suspect that to be an individual going to fast.

Ha! You have to be kidding? There are so many ways to do testing wrong. But the point is, if you do it a little wrong the exact same way every time, the consistency will at least show you changes in your tank rather than changes due to the way you do the test. If you do it wrong in a big way, maybe you need to look at your ability to do the tests at all?

[CStrickland]

I think that by using a volume measurement, you are introducing an uneccesary variable into your test. Salt can be compacted or fluffy etc based on settling and humidity and other issues. I would try again, this time mix the water to a defined specific gravity as measured on a calibrated refractometer, then test the levels of minerals.

Once you figure out how much salt it takes to get your desired sg, it's ok to use volume measurements to make batches of salt. But it's still a good idea to confirm that you are hitting your mark. Some people weigh it too, which is better than volume but for this sort of test still not as good.

That said, your variances are within the range of our tests IMO. There's some threads in the Chem forum that compare stuff like this, people are frequently surprised how much of a difference there is between and among tests. I suppose because it's not something we really look at that much.

[Ron Reefman]

Our local club did a very unscientific test. Everybody (about 40 people)brought their test kits and we all tested the same sample of water. The results were all over the map but did tend to clump more at a midpoint. The water sample was sent out for a professional analysis and most of the test kits were in the ballpark, but even the first standard deviation was significant.

[2_zoa]

Originally Posted by Ron Reefman

Ha! You have to be kidding? There are so many ways to do testing wrong. But the point is, if you do it a little wrong the exact same way every time, the consistency will at least show you changes in your tank rather than changes due to the way you do the test. If you do it wrong in a big way, maybe you need to look at your ability to do the tests at all?

No, I was serious. Maybe you could give a few examples of how to do a test wrong that I'm not thinking of? My examples were of a pretty gross error. Too. many drops, not enough drops, etc.

Originally Posted by Ron Reefman

Our local club did a very unscientific test. Everybody (about 40 people)brought their test kits and we all tested the same sample of water. The results were all over the map but did tend to clump more at a midpoint. The water sample was sent out for a professional analysis and most of the test kits were in the ballpark, but even the first standard deviation was significant.

So, it would seem that. Test variance is more likely to happen then a gross error on the users part, no?

[otimer]

Thanks everybody for the answers and comments.
Sorry for my silence. I was in a situation that I could not write during the weekend.

Yes, calcium is not a big problem as it is already in an adequate range.
But for the magnesium, I should dose big amount of magnesium supplements (magnesium Cloride and Magnesium Sulphate) initially to the tank as well as for every water change, if I take data from the current test kits as references.

The test work is very simple and I 'm not a new comer in this hoby, so there is no possibility to make mistakes in the testings. I tried in many different ways such as dropping slowly, shake shorter/longer, and fast/slow titration. They didn't make big difference. Also I discussed on the proper way for the testing with local reefers, but they all do in a same way. They say different results for same water with different kits. But not so much difference like my case.

One thing I did not describe is that the test kits were stored in a room which has no air conditioning. And it was really hot in my country in this summer. The temperature in that room probably went up to about 37~38'C (98.6~100.4'F) for the last three weeks. I could observe the difference of colour of the powder in the calcium test kits. One is more redish than the other. I'm not sure this high temparature could change the characteristics or degrade the test kits.

Accordingly, I'm planning two ways to verify the descrepancies. I will buy new test kits from different manufacturers. Also I'm studying how to prepare calibration solutions to veryfy the test kits. It could be possibe if I find how to eliminate moisture from calcium Cloride (CaCl2). Then I weigh exact amount of that with a pricision scale and mix with 1 litter of RO/DI water. This can make about 400ppm solution. In a similar way, it is probably possible for magnesium as well.

Does any one have better idea or experience in verification or to prepare reference solutions?
I will write the results if I scucced.
Thanks again!