I don't recommend adding iodine for a variety of reasons:
1) a ton is added via food
2) it is nearly impossible to test for the different forms of iodine accurately, especially since it speciates into various molecules in seawater. A lot can also be complexed in with various organics.
3) Shrimp and crustaceans in general, contrary to what is believed, do not do better with added iodine. In fact, they go through great lengths to remove what is absorbed from the water out of their bodies via molting. This is also why they molt more frequently with adde iodine--it is a toxin, it isn't helping them. Actually, they get more than enough iodine for various processes, etc. via their foods. This isn't going to be that big of a shocker, but the research showing that crustaceans use iodine to molt (as well as an adaptation to get rid of iodine) was referring only to dietary iodine, not dissolved iodine.
In the end, I agree that any needed iodine is supplied from foods and salt mixes. There really isn't any other need for it and there is little evidence that things actually need it, other than from their diet.