On the other hand, Wright Huntley, another old-timer in the hobby, in another salt discussion, has suggested

"Iodized salt is OK, but premium brands use silicates to make them free flowing. Those seem to clog gills, so use the cheap generic grocery brands that use "yellow prussiate of soda" instead. Rock salt, pickling salt and some kosher salts have no additives -- just pure sodium chloride. They are good, but harder to measure with the larger crystals, sometimes."

He has had no problems with fry dying with those salts.

As for iodide in the salt, a tiny bit is needed by living things. Guppies and virtually all livebearers are secondary freshwater fish, meaning that scientists feel that their ancestors "wayyyyyy back when" were marine fish. They should be tolerant of a little sodium chloride and iodine.

A friend and I, visiting an aquarist who had some fish with a glandular swelling under their gills, suggested that it might be "fish goiter". We recommended putting a little iodized salt in with the fish (which were killies accustomed to hard water). By the time we left a couple of hours later, the swelling on the fish was down and they were swimming normally.

I would suggest if that Yellow Prussiate of Soda was safe for humans (who will be running it through their digestive system), it is likely safe for fish. There are so many other salts available, they are certainly can be avoided.

If I needed it in a hurry and the only salt I had was an inexpensive bag of table salt with the Yellow Prussiate of Soda; I'd use it. :)

If the salt clumps, it is probably packaged without anything to make it free flowing. The salt from the 50 Lb. bag of cattle feed salt (probably sun dried sea salt and therefore likely containing useful trace elements) we picked up ages ago, cakes. So it is without additives. It only costs a few dollars from farm supply stores.