Confusing test results?

[Chortanator]

Hi all,

I've just started regularly testing my water with the Red Sea kit for Mg, Ca, and KH. I'm trying to test as often as I can, but I'm getting results that I don't really understand. These could be user error, and I'll keep trying to test as frequently as possible so that I can get some consistency in my results, but I'd just like to know if these seem like something that could occur without user error:

May 11
Salinity: 1.025
Mg: 1400
Alk: 9.7dkH
Ca: 450
Nitrates: 5

May 12
Salinity: 1.025
Mg: 1300
Alk: 9.1dkh
Cal: 400
Nitrates: 3

May 15
Salinity: 1.025
Mg: 1300
Alk: 8.85dkH
Cal: 425
Nitrates: 2.5

I don't understand how my Mg can drop 100 points in one day, and then stay stable for 3 days after that. I also don't know if it's possible for Ca to go up on its own? I haven't dosed anything at all, as I was trying to do the 4-day testing that Red Sea recommends to see what my tank uses on a daily basis (which, clearly, I forgot to do, but I'm going to try again after my next water change). But I also have no idea how much I should dose.

I use Red Sea Coral Pro salt, mixed to 1.025 salinity.

 

[MnFish1]

1. I did not take the time to look up the 'margin of error' of each test - so I would take that into account.
2. If you compare the second 2 tests - they are basically identical - it is the first test that seems 'different' - and its not 'that' different'.
3. I might try to verify with another test kit - and also consider the time of day you did the tests - were they all the same time of day?

 

[Chortanator]

Here is the margin of error as per their website, so my Ca test is 20ppm outside of that. I don't have another test kit to verify, but it's my understanding that the Red Sea Foundations Pro test kit is one of the most accurate on the market. I only bought it last week, and I'm not sure if my LFS even carries anything else for Ca/Mg/KH. And yes, the tests were all done at roughly the same time in the early afternoon.

 

[MnFish1]

Here is the margin of error as per their website, so my Ca test is 20ppm outside of that. I don't have another test kit to verify, but it's my understanding that the Red Sea Foundations Pro test kit is one of the most accurate on the market. I only bought it last week, and I'm not sure if my LFS even carries anything else for Ca/Mg/KH. And yes, the tests were all done at roughly the same time in the early afternoon.

OK - thanks for looking that up.

My problem with most of the tests - including Red Sea - is that the colors are often so close - that even a change of light or the way you're holding the vial etc - can change the results. But having said that - and assuming that you did them correctly - from a 'tank perspective' - the only result I would be questioning is why the first alkalinity was so high at 9.7 and the last was 8.85. (again assuming your tests are accurate)

So -

1. Did you add anything to the tank between the first and last test.
2. Realize that levels change from day to day - and I don't think especially the results between test 2 and test 3 are significantly different.

But - it seems to me that the chart you're using - is really measuring the 'precision' of the test - as compared to the accuracy of the test. (it looks like they did 75 tests for each one). Precision being how close test 1 to test 2 is to test 3,4,5 etc. Accuracy refers to how close the test result is to the 'real value' - measured by a gold standard,

Here is some more information from RedSea - I'm not sure which kit your using - but the bottles and the instructions look identical between the single Mg test kit - and the other one??:

"

Accurate measurement of Magnesium in marine aquariums​

Mg

Red Sea’s user-friendly Magnesium titration test provides a quick and reliable measurement of Magnesium in your aquarium to an accuracy of either 50ppm or 100ppm according to your needs.
Kit includes easy to follow graphic instructions and conversion tables for immediate interpretation of results.

Red Sea Magnesiun Test Kit Includes 75 tests.​

 

[Chortanator]

I'm using the Red Sea Foundation Pro kit, which comes with Mg, Ca, and KH. I believe that the 75 tests on the chart I provided is listing how many tests are in each kit. I didn't add anything at all into the tank between the tests.

Also, the reason why the alk is so high is because I'm using the Red Sea Coral Pro Salt (black bucket) which mixes at 11-13 dkH.

 

[Randy Holmes-Farley Verified Community Expert]

Unless the aquarium was greatly diluted between the magnesium tests, the change is just test error.

The real mag drop in your situation is well below 1 ppm and is undetectable by kit.

 

[Chortanator]

Unless the aquarium was greatly diluted between the magnesium tests, the change is just test error.

The real mag drop in your situation is well below 1 ppm and is undetectable by kit.

Thanks for your reply Randy. I'll admit, that first test was the first time I had ever done the Red Sea mag test. As for the Ca, do you think that's an error too? Or is there any reason why the Ca in the tank would go up without any dosing?