Using a good salt or nsw to do a few water changes is best to raise your levels, and is also more balanced than suppliments. (In my severely lacking scientific understanding I would think adding chloride and sulfate in a balanced ratio would still cause these two elements to be unbalanced {in excess} with other ions???) Typical additives only raise your Mg level by 5mg or thereabouts per dose! So you gonna need a whole lotta suppliment to raise it. Also, according to seachem, all liquid Mg additives contain ammonia, so you will have to add it slowly to give your system time to break down the ammonia!
Using the calciumreactor with aragonite as opposed to synthetic media does help to maintain MG, but I have found that it does drift slowly downwards. Adding magnesium media as per allan will definitely help quite a bit!
Why is ur Mg so low? What is the sg of your water? Are you using Kalkwasser for top ups, as this does lead to Mg depletion!