Besides iodine, the bad chemical in table salt is prussic acid (yellow soda of prussate) You can buy a big container of morton plain salt for 59 cents and use that. It contains calcium silicate, which is a mineral found in most tap water and is harmless for fish. That's the kind of salt I use. I was just wndering if you fish had ich or something. I know a few people keep a permanent salinity in their tanks, but I usually change it out after the fish are ok.

On a side note, I also switched to prussate-free salt. Apparently in its liquid form it is extremely toxic to people. In the form used for salt, tests indicate no harm to humans, but there is still harm to smaller mammals. I'd just rather not put extra, questionable chemicals into my mouth.