Using iodized salt will only cause a problem if you put in enough salt to pickle the fish.
Iodine is needed by fish just as it is by humans. The amount in table salt is trivial. You can safely use table salt. As long as it's NaCl, you're okay. There may be other additives in some salts that aren't great, but iodine isn't one of them.
And, yes, I use table salt for my fish, when needed for medicating or treating high nitrite levels. Heck, many moons ago I used to use 4 tsp/gallon table salt with my molly tank--big, beautiful fish. Healthy as the proverbial horse. And, they never had goiters.
Let's not encourage myth. If you 'hear' something, always investigate it to verify if there's logic/facts at work, or just an oft cited statement.