As folks mentioned, just be sure to not try to raise magnesium with this or any product in a CaCO3/CO2 reactor. 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.