Water tests dont match ICP


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May 22, 2022
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New to this, so still trying to understand.
I have been doing Calcium and Magnesium tests almost daily. The tank is 3 months old with 6 young LPS corals.
I was watching calcium, magnesium and dkh for shifts so i can understand what I need to dose.
dkh started to drop and over the last 3 weeks or so dropped from 8.4 to 8.0

However i noticed both magnesium and calcium were remaining high. Both of my trachy's are unhappy about something, so i know something is wrong. So i submitted an ICP test.
Calcium came back as 384mg/l and magnesium at 1150mg/l
Both low.
I am using the red sea pro test kits for magnesium, alkalinity and calcium.
Magnesium is coming back as 1480mg/l
Calcium is 450mg/l

This is quite far off the ICP report. And I trust the ICP report more as I have not had any discernible variation in test results with the red sea kits. There should be some consumption.
I calculated what the syringe needs to read to match the ICP report and tried to dispense (slowly) reagent to that point. But twice the results have over shot, by a long way.
So I tried another test kit.
Nyos Calcium. Basically the same titration process as the red sea, result: 450mg/l

Why am i seeing such large differences between ICP and my own tests?

It get worse with alkalinity.
Hanna checker shows 8.2 today (I am slowly dosing to bring it up)
Red sea titration is 7.0

I assume I am doing something wrong here. I have watched the Red sea videos on youtube that show you how to use the tests. I am following them correctly to what i can see.
I do my tests during the day and read the results outside just to be sure i am not reading the colors wrong.

Any suggestions or tips?

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Mark Novack

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Mix up a batch of new water. Test it. See if the results compare to the numbers on the salt mix. I use Red Sea and it gives a min/max of elements that should be present. Test numbers should fall between there. You can send a sample of new water to ICP also.

On the subject of new tank, I recommend that you get your magnesium up over 1350 before you fight an endless battle with calcium. Mg is the key in the beginning and is connected to calcium levels. I don't know why, but magnesium disappears in new tanks. I have always needed to add a lot in the first months and then very little thereafter. Calcium cannot be stabilized without enough Mg.
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