Lighting can play a major role in zoa health. Especially if your lights arent giving off enough par, and the right spectrum. I know it sucks, but id look into changing your bulb combo. Than at leat youve eliminated that as the problem. Ive never dosed iodide so.i cant tell you anything in regard to that.. if there turning brown that could be the zooxanthelle which happens alot with low lighting. Hows your N03 and P04?