Aquarium salt has no iodine in it. Iodine was added to salt to prevent the medical issues that many people has with lack of iodine in their diet. But it is not good for fish.... so no iodine in fish salt...
non iodized may still have some ingredients to make it pour easily... not sure what they add. You can check the label.
You would have to look on the label to make sure there are no anti-caking agents in whatever salt you pick up. Like everyone above said make sure it has nothing else added to it and that it is 100% pure salt with no anti- caking agents.
Maybe someone can link brands to you that they have used successfully, otherwise API Aquarium Salt isn't really pricey and comes in a small container that lasts forever if you are just using it to treat the occasional illness in your betta.
Everything by me marketed as "Ice Cream" anything tends to be crazy expensive- In your area this may or may not be true. I am not familiar with grocery store salts you can buy though.
If people could link safe brands it would be nice though. I went with API because I didn't want to get the wrong salt and kill my fish.