Ca/KH relationship

[Spongebob]

While I realize that both Ca and Alk levels should be kept up at around 420 ppm and 10 dKH, I am not sure how the two interact. Does an increase in one affect the other? Does adding an Alk booster to my tank decrease the Ca level or vice versa? Does the level of one limit the other? Thanks for your help!

[Randy Holmes-Farley]

. Does an increase in one affect the other?

It does if they rise too high. Otherwise, the do not.

Alkalinity measures a variety of things, but mostly bicarbonate (HCO3-) and carbonate (CO3--). As alkalinity rises, both of these rise.

Calcium and carbonate will precipitate as calcium carbonate if either or both rise too far.

Since seawater is already supersaturated with respect to calcium carbonate, there isn't much room to dissolve more than the seawater can hold, so rises in either can quickly lead to precipitation.

Here's a thread where I discuss this in simple terms.

http://www.reefcentral.net/vbulletin/showthread.php?threadid=60012

In actuality the situation is very complex. Even other ions like phosphate and magnesium impact the balance between carbonate and calcium. So does temperature, pressure, and a host of other things.

This article discusses the magnesium contribution:

http://www.animalnetwork.com/fish/library/articleview2.asp?Section=&RecordNo=2856