seems like reefkeepers like Lugol's:


http://www.reefs.org/library/article/e_edelman2.html


Commercially available Iodine supplements generally use Potassium Iodide and several of them use a chelating compound such as EDTA. A buildup of EDTA may be harmful to your reef, the jury is still out on this one, but many hobbyiests try and stay away from chelated compounds.
Lugol's solution is made up of 5% KI and 10% I2 by mass, in water. Many people have switched from commercial Iodine preparations to Lugol's solution for economic reasons. A bottle of a commercial preparation may last you six months and cost $10. A pint bottle of Lugol's costs approximately $15 and will supply you with Iodine for the rest of your life.