Magnesium usually only impacts the calcium so I don't think it is low, however everyone is correct, you need to test for it.
Your sitution is very similar to mine. I don't run a calcium reactor and I drip kalk but my Alk drops about 1 dkh per day.
Your alkalinity is low, you can raise it by adding sodium bicarbonate (baking soda).
If you raise alkalinity too much it will drive down calcium, there is a balance between the two - too much of one drives down the other.
Because of this I am now using the http://www.bulkreefsupply.com/a5/Recipe-2-Instructions-for-Bulk-Packaged-Materials/article_info.html">Bulk Reef Supply 2-part recipe</a>. Their bulk calcium and magnesium are very affordable.
Alkalinity: Mix 1-1/8 cups of baking soda per gallon of RO water
Calcium: Mix 1 cup of calcium chloride per gallon of RO water
They have a good [IMG]http://www.bulkreefsupply.com/i6/Calcium-and-Alkalinity-Calculator./information.html">reef calculator </a>that will determine how much you need to dose.
Once your levels are correct, dose equal amounts of each part of the recipe. The key is to dose equal parts of each that way you don't drive down one component or the other.
Give me a call if you want and I can explain further. My system has been real stable since I set up this routine. 678-283-0031
Heath