AFAIK, its a myth that iodized table salt is harmful.
I've used iodized table salt with all those anticaking agents numerous times almost as high as 3 teaspoons per one gallon, i put ~130 teaspoons of salt in a 55g to treat for ick. No ill effects. The tank was treated for 3 weeks and had "salt sensitive" fish in it at the time. They all made a 100% recovery.
i agree i have read countless articles in fish rags and web published that table salt is fine in fact some experts think that table salt is better when treating sick fish
so just use the salt they have to put in salads , i have the table salt and the one
that is put on food cooking the thick one ,is 100% natural ,they must have that in there