Over what period of time?
That may be testing error. All reactor media contains some, but many do not have enough to meet the demand in the tank. None can be used to boost magnesium if it is already low without too large of an increase in alkalinity.
I discuss that here, with the same issue applying to dolomite as normal media with magnesium in it::
Magnesium in Reef Aquaria
http://www.advancedaquarist.com/issues/oct2003/chem.htm
"It has been suggested that adding dolomite to CaCO3/CO2 reactors can help with magnesium problems. Dolomite is a material that contains both magnesium and calcium carbonate. If dolomite is being added to the reactor to maintain existing appropriate magnesium levels against the continual depletion via calcification (for example, if the calcium carbonate being used is too low in magnesium to maintain adequate magnesium) then this is a fine approach.
However, this method is unsuitable if the goal is to raise magnesium levels. The problem is that for every magnesium ion released from the dolomite, 2 units of alkalinity are also released:
MgCO3 Mg++ + CO3--
Consequently, if one wants to raise magnesium by 100 ppm, the alkalinity will necessarily rise by 8.2 meq/L (23 dKH). The only way around this problem is to add a mineral acid (not vinegar) to the aquarium to reduce the alkalinity, and that may be more problematic than just adding magnesium in the first place."