Common table salt which we use in our food is NaCl with some amount of iodine and other useful additives. Many of us had fishes who got suffer with some parasites infections like itch and salt treatment method is extremely help full to treat fishes with itch infections.
But as i have already told that table salt contains iodine and many people believes that iodine is extremely dangerous to aquarium fishes but on the other hand many people also believe that it is harmless. Actually there is controversy over this. So it is recommended to use kosher salt if you don't want to take risk. However i would like to share my personal experience with you all people.
My personal experience :
I was having 2 rosy barb and few black skirt tetras. Everything was working fine then i decided to brought 2 more tetras to the same aquarium. I went to aquarium dealer and bought 2 more. Without noticing them i added them to main tank and what i found they were having white spots all over their body and in next few days i found that each and every fish are having white spot and those poor fishes were rubbing with each other and also with filters trying to get some relief. Then i went through google and decided to add iodized table salt because it was present in the kitchen. I couldn't resist my self. I added table salt, i added 1 spoon per gallon and raised aquarium temperature to 32 degree Celsius. But everything was fine and after 4-5 days i found that itch had been disappeared and every fish is living fine till now. So finally i would suggest that you can add it at your own risk or may be i am lucky.