Magnesium dropped.

Longnose Hawkfish · Oct 9, 2014

Hello everyone, I have a 93 gallon tank about 2-3 months old and it contains probably 30-40 small frags. I tested my water two and a half weeks ago and got these results.
Mag 1480
Calcium 420-430
Alk 8.5

I tested yesterday, so two and a half weeks from the last tests, and these are my results.
Mag 1200
Calcium 410-420
Alk 8.5

Why did only the mag drop? Thank you all!!

Randy Holmes-Farley · Oct 9, 2014

Unless you did a massive water change with a very low magnesium mix, it cannot have dropped and one or both of the tests is in error.

There is just no way for magnesium to drop like that. Unless you added massive amounts of calcum and alkalinity (hundreds of dKH and thousands of ppm calcium), you cannot remove that much magnesium through precipitation or consumption.

Longnose Hawkfish · Oct 9, 2014

Ok thank you. I will do a re test on the mag. Does my other levels make sense?

Randy Holmes-Farley · Oct 9, 2014

Yes, the other numbers look perfectly fine.

Are you presently dosing anything?

Longnose Hawkfish · Oct 9, 2014

No I am not dosing anything. I have only dosed mag and alk once and that was to bring the numbers up before I added any corals.

Randy Holmes-Farley · Oct 9, 2014

Given that, the magnesium almost certainly matches what came in with the salt mix.

What mix are you using?

Longnose Hawkfish · Oct 9, 2014

Not sure on what I used to set up the tank with. A friend helped me set it up and he used that salt mix and he loves it. I just switched to Red Sea coral pro.

Randy Holmes-Farley · Oct 9, 2014

Longnose Hawkfish said:
Not sure on what I used to set up the tank with. A friend helped me set it up and he used that salt mix and he loves it. I just switched to Red Sea coral pro.

OK, I can't comment on what was in there before, but Red Sea Coral Pro has magnesium numbers more like your fist reading, if you are at 35 ppt (sg = 1.0264). Instant Ocean has numbers more like the second reading.

Longnose Hawkfish · Oct 9, 2014

Ok thank you very much. I know it wasn't instant ocean.

Longnose Hawkfish · Oct 9, 2014

Just tested again and got 1280.

Randy Holmes-Farley · Oct 9, 2014

Well, that exactly matches NSW at 35 ppt (sg = 1.0264) so it is not any sort of concern, but if you want to raise it, I'd suggest a DIY magnesium supplement.

Or you could just wait to see how it evolves with the new salt mix.