Two articles on iodine:
Iodine in Marine Aquaria: Part I:
http://www.advancedaquarist.com/issues/mar2003/chem.htm
Iodine in Reef Tanks 2: Effects on Macroalgae Growth:
http://www.advancedaquarist.com/issu...l2003/chem.htm
Personally, I don't test for iodine, though I do test for alk, Mg and Ca regularly and adjust levels when they are off. If lack of iodine is the limiting factor in the growth of some organisms in my tanks, I accept that, especially after reading the first article.