Only consuming Mag?

[Shazbop07]

My tank has only consumed magnesium since June. Alk and calcium stay level.
Any ideas why this would happen?
Last parameters this week:
Mag 1290-dosing 10 ml aquaforest mag plus BD on dosing pump
Ca-455
Alk 10.7
No3 16-biopellet reactor
Po4 0.04-gfo reactor
SG1026
4ft waterbox 390 litres roughly. Mainly softies, couple small acros, acans, hammer and torch.
Water changes weekly 10% with NSW.
RO water
Some coralline growth but no mass explosion to explain where mag is going
Ideas?

 

[Randy Holmes-Farley Verified Community Expert]

Test error is the only possibility (aside from water changes with a low magnesium mix). There is no mechanism for removing magnesium that does not also remove about 10 times as much calcium and a lot of alkalinity.

 

[Shazbop07]

Thanks Randy. I think what's happened is back in June I had an alk spike up to 12.
I've had water tested at lfs and its correct. New test kits (salifert) , redo tests 3 times, rinse with RO etc.
I cant get alk down to where it used to be at around 8-8.5, water changes with nsw of alk of 8 , it bounces back up to 10.7 next day. Im going a bit crazy trying to figure out why and I'm dubious to add coral as it will just die. I've lost quite a bit of coral since June. I've asked every experienced reefer I know, they all scratch their heads... I have white spots on back of tank which I assume are precipitated ca+?
I'm doing 10% nsw weekly changes now to see what happens and not to dramatically alter the chemistry too much. Dosing af mag plus 10mls twice a day.
I greatly appreciate any ideas/advice

 

[Randy Holmes-Farley Verified Community Expert]

Just to keep perspective on theories of magnesium decline, let’s look at the most mag that can possibly be removed even if youvorecioitated pure magnesium carbonate (it won’t, this is just an extreme thought experiment).

Suppose that you want to remove 240 mg/l of magnesium this way. Since magnesium has a molecular weight of about 24 g/mole, that is 0.01 mole per liter, or 10 millimole per liter. That much magnesium needs 10 mMole/l carbonate to make magnesium carbonate. That is 20 meq/L if alkalinity, or 56 dKH of alkalinity. Thus, No spike to 12 dKH is going to noticeably deplete magnesium.

In reality you likely need over a thousand dKH to take out most of the magnesium in seawater.

 

[Shazbop07]

Ok. Thankyou for replying on a weekend! I understand that. So what would you do then?
I'll take some water in to lfs today where they have Hanna checkers and see what it shows. Think I may slowly invest in some myself

 

[Randy Holmes-Farley Verified Community Expert]

Ok. Thankyou for replying on a weekend! I understand that. So what would you do then?
I'll take some water in to lfs today where they have Hanna checkers and see what it shows. Think I may slowly invest in some myself

I don’t think there is a Hannah checker for marine magnesium, but the lfs probably has some kit to test it.

 

[Shazbop07]

I have fixed my issue. Thanks Randy for your advice. I took water to lfs. Results on Red Sea kits were
Alk 7.8
Ca 450
Mg 1450
So considerably better than what I was getting.
Oops Yeh there is no hanna for mag!
Salifert kits in the bin. Have bought red sea kits instead. Will work out my consumption now and start again

 

[Randy Holmes-Farley Verified Community Expert]

Sounds good.

Happy Reefing!