Let's get something clear here. Table salt does not contain iodine. Table salt contains potassium iodide. Iodine and iodide are as different as chlorine and chloride. Chlorine is a toxic gas used during WW1, chloride is the relatively harmless ion present most famously in table salt as sodium chloride.
Iodine (I2) is an antiseptic often used to disinfect wounds, iodide is an ion necessary for proper growth of all vertibrates and required by inverts like shrimp for moulting.
Yes, there are sites that report that table salt contains some magical compounds that are bad for your fish. This would be forgivable if it could be passed off as ignorance, but coming from a site trying to sell you "aquarium salt" should be the first thing to tip you off, the deliberate spreading of misinformation by these sites is only less than infuriating.
But before we get worked up about all these evil compounds in table salt, let's have a closer look at what and how much is present, shall we?(
http://sci-toys.com/ingredients/table_salt.html)
For one thing, jumping back to this horse manure about iodide toxicity, there's not nearly enough KI to be even remotely harmful to your fish, at the levels present it would probably even be beneficial to your tank.
As for the dextrose or other stabilizing agents, well, firstly, there is so little there that again it is negligible, and if you're getting nervous about sodium thiosulfate, do a DIY forums search on DIY dechlorinators.
The scale makes the difference, and it's huge. Table salt ranges from 95-99% pure NaCl, it's dirt cheap. Now some folks want to sell you on "pure" NaCl, package it up and call it "Aquarium Salt", it's 99.9% pure.
There is
no difference between the two. period.