I still find that I have to dose Mg. every once in a while as mine will creep down below 1300. I have been using the Seachem Mg. but here is a link to a page that lists what I bought (it is the granules)
http://www.captiveoceans.com/product...pplements.html
Here is what it says about them:
ZEOmag is a pure magnesium granulate to be used in calcium reactors (5-15% of total media volume) to maintain magnesium levels. Magnesium is easily added without chloride and without disturbing the ionic balance in your tank. Granulate size 6-12 mm.
The tank that I have to dose the most often is my cube tank that does not have a reactor hooked up (yet). What I usually do is just add Mg. to my change water to bring the level up to just over 1400 in that water and it seems to keep things fairly steady in my tank.
I think that the most advantageous thing about a calcium reactor is that it keeps your levels steady and consistent and balanced and not constantly changing the way they have a tendency to do unless you are super diligent in adding exactly the right amount of 2 part or other solution.