Knowing the Magnesium level in your reef tank will allow you to know exactly how much to dose to achieve the desired Mg level. 

The Elos Magnesium Test Kit includes four reagents, a glass vial with cap, a 5ml syringe, a plastic dosing spoon and instructions in several languages.

All salt mixes contain Magnesium, but this element depletes over time and needs to be replenished. For example, Montipora sp. loves Mg and keeping it at 1300ppm or higher has always worked best for mine. If Magnesium is overdosed, it may cause snails to cease movement or fall off the glass. Testing weekly is the best way to know if the tank needs more, or not.