Myth it may be, but I practice iodine addition as well. If you try rearing Amano fry without it I think you will have problems (lots of molts there), or just maintaining some of the showier Macrobrachium shrimp (which used to be a problem for me). You either use iodine-rich or iodine-supplemented foods, or I can neither rear nor maintain through serial molts. My tap water BTW is GH ~9, KH 7.