Magnesium chloride. You can add 10% Magnesium sulfate in addition to maintain a better ion balance.
Make sure you test yur Mg level before adding any though. Adding about a pound (yes, a pound) will raise the magnesium level of 100 gallons of water by 100ppm. Target somewhere around 1300ppm.
Don't add this just because you've heard soft corals need it. Only add it if it's low. To find out if it's low you will need a Magnesium test kit.
If you don't test for it, don't add anything. You would be better off just doing larger and more frequent water changes than dumping chemicals in untested.
