Two Little Fishies Magnesium Pronto is a fast-dissolving blend of pure anhydrous magnesium chloride and magnesium sulfate that makes it easy and economical to rapidly raise the magnesium ion concentration in aquariums. Supplemental magnesium is often required for reef aquariums because corals, clams, snails, tube worms, and calcareous algae extract it from the water. Mix up the correct dose per the instructions listed on the label.
melev's recommendation: I mix up almost a gallon of solution at a time. This jar can make about 5-gallons worth of liquid magnesium for your aquarium. The exact ratio I use is 1 tablespoon of Magnesium Pronto per 12 ounces of water. A gallon is 128 ounces, but you need to have room in the container to add the powder and mix it, right? So instead, I suggest you use 120 ounces of RODI water. Put that in an open top container, and add 10 level tablespoons of Magnesium Pronto to the water. Mix it well by stirring it with a wooden spoon (never use a metal spoon), and let it sit for a few minutes until it's completely clear. The solution will be rather warm during the mixing process; this is completely normal. Make sure the container you used doesn't melt from that heat.
Now that you have a jug of clear liquid magnesium, you can pour that into a dosing reservoir if you own one, and with a dosing pump trickle in the desired amount necessary on a daily basis. Test your Magnesium levels frequently to avoid overdosing the aquarium. If you are dosing by hand, add the pre-determined amount in an area of high flow very slowly so it can mix into the saltwater effectively.