I use Kent's Tech-M Magnesium and buy at least two of the big jugs from J&L when ordering dry goods so I don't run out. I drip kalk in all three of our reefs, which helps sustain Ca and alk levels, but does nothing for Mg levels. Sure use a lot of the stuff
Anyway, it's a liquid and easy to measure with a 10 ml plastic syringe from the drug store. Make sure to shake liquid Mg thoroughly while using as the heavier particles settle out quickly.