While I am going to quote RTR here. This man has been keeping fish longer than most of this forum's members are alive and has more than 30 tanks active right now. If you want to read the full article that this quote came from, click on the link that Roan Art posted. Roan, myself and countless other forum members have used table salt without any ill affects on the fish.
"Iodine is a halogen, and is required for vertebrates. It is necessary for our metabolism as an essential part of thyroid hormone, which is our metabolic pacemaker. Soils in wide areas of this country are deficient in iodine, and can result in goiter (hypertrophy of the thyroid gland, effectively from insufficient iodine intake). Thus the practice arose of adding iodide to salt intended for human consumption. This was the safest (the levels of iodine are minute) and surest way of protecting the population from this deficiency as salt is ubiquitous (all but universal) in food processing and preparation. The levels of iodide added to table salt are so small that any water-living vertebrate would be pickled in brine well before toxic concentrations of iodine/iodide could be reached, so that particular urban myth is without foundation. In fact, a number of our tank inhabitants need iodine- most crustaceans have a significant demand for the material, and a number of fish can develop goiter in captivity from the lack of iodine- African Rift Lake fish seem especially prone to this. The often-discussed toxicity of iodine could be considered urban myth #1. "