If you are going to be using kalk you should consider monitoring magnesium over shorter periods of time, i.e., 1 / wk or 1 / 2 wk. All sup have their issues, to include Magnesium Sulfate, Magnesium Chloride, Calcium Chloride additions, all which raise the Sulfate an Chloride levels and require even more frequent WC.
What happens is when the kalk hits the water the immediate vicinity at the kalk water interface will precip some CaCO3 as Calcite. Mg++ fits real nice into the Calcite producing what we call High Magnesium Calcite, which is no all that soluble. There is also some Magnesium Hydroxide, Mg(OH)2, which is that cloud you see at times but where most of it goes right back into solution.
I might add, that as a sup, Kalk benefits by far out weigh the others as a Ca++ sup. It does not increase the Sulfate, it does not increase the Chloride, it raises pH, it raise Alk, it "eats" CO2 and there are no byproducts, to include excessive Sodium or excessive Bromide. Most advanced reefers use Kalk reactors or Calcium reactors but these are usually in tanks with high demands such as SPS tanks. It even adds at reducing phosphates to a small degree. You must also remember that Mg++ is used by organisms, so it is going to deplete naturally. If someone is having excessive problems with kalk over a short time, where Mg++ is depleting you are then doing something wrong.
In a properly run system with kalk, with kalk properly mixed and siphoned off the top, so you do not get the crap on the bottom, no animals or plants using it and no Mg++ sup, no WC, where there is a daily addition of Kalk, that is equal to 2 % of the of tank volume and the starting Mg++ 1300 ppm, the drop in one year will be ~150 ppm .
To add or you have a bad test kit. Mg++ CAN'T deplete anywhere near the same rate as Ca++.
