Magnesium consumption

[UltraKelius]

I’ve been doing daily testing of Magnesium to get dialled in on how much to dose daily to stop the swings. According to the results I’ve had so far over the last four days, my 20g mixed reef is using up 30ppm daily.
Does this seem a little excessive to anyone? Be interested to hear what other people have tested their tanks consumption at. Tested using Salifert kit.

 

[Randy Holmes-Farley Verified Community Expert]

That's just test error or salinity changes.

Magnesium declines are about 1/10th of the calcium decline (never more), and not generally more than about 2 ppm per day even in a high demand tank.

 

[nereefpat]

Mg consumption by corals is about 10% of Ca. So 300 ppm of Ca... and about 40 dKH since that's about 7:1 with calcium.

I think you're seeing noise in the Mg kit.

 

[UltraKelius]

Oh wow, that is interesting. So are Salifert Mag tests known to be that misleading then? My salinity is usually rock solid apart from a couple of occasions where my skimmer has pulled out a bit more than I wanted. Calcium last tested at 430ppm six days ago. When I tested Magnesium on the same day it was 1275ppm.

 

[nereefpat]

So are Salifert Mag tests known to be that misleading then?

It is a good kit. It used to be about the only hobby one available. It does have an error, like any test kit. I can't find what it is, but probably about +/- 30 ppm. So if you get a reading within about 30 ppm of the last one, it may be that the value is the same rather than trending downward, for example.