Relationship between Ca and Mg

[cgkrick]

I just started measuring and trying to control Mg in my tank. I am trying to do a better job with Ca, Mg, and dKh because I am moving to LPS.

I was at Ca 420 and Mg 1280. Added enough Mg additive to bring Mg to 1300. My next measurement was Ca 480 and Mg 1420. I am using Salifert test kits and I am a chemist, so I think my technique is good. DKh was 10 before and after.

Would an increase in Ca be expected just from adding Mg?

 

[reefer gladness]

In short, yes. Low magnesium reduces the solubility of calcium and buffering capacity.

For the chemist in you here's a good read explaining the complex (at least to most of us) relationship going on between calcium, alkalinity, magnesium and pH.
A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH by Randy Holmes-Farley - Reefkeeping.com

 

[Choff]

I read many good articles on the subject, the one reefer linked being one of the best.

This video from the bulk reef supply guys is what finally got it to sink in for me.


http://www.youtube.com/watch?v=mI52IyBtjp0&feature=youtube_gdata_player


Sent using Tapatalk 2

 

[Inverted]

Calcium and "alkalinity" (actually, carbonate and bicarbonate) want to precipitate out of the water and form crystals such as aragonite and calcite. Basically sand, limestone kinda stuff. The amount of calcium and "alkalinity" the water can hold though, at healthy pH levels isn't much, compared to what we need. So, we actually need the saltwater to be supersaturated with ca and alk. Luckily when these precipitate crystals form, magnesium also binds to them and "poisons" the surface preventing further growth of the crystals and the ca and alk has no where to go. So, remains in the water, allowing more than there should be without the magnesium. Also, other metals can have a similar effect, phosphate for example, but these are less desirable as they cause other complications.

 

[Inverted]

By the way, thinking about the last part "Would an increase in Ca be expected just from adding Mg?". I'd have to say no. The only exception maybe would be if pH dropped enough to dissolve some aragonite, for example. Then perhaps it would be less likely to re-precipitate even after the pH increased. However, to really dissolve any significant aragonite, the pH would need to get down below say 7.5ish and then it't not the mg raising the ca, it's the low pH. At that point though, you corals may also start to dissolve, as their skeletons are aragonite. well, the can probably go a bit lower as internal pH is regulated to an extent, but still....