Although this thread is old I just wanted to add a few things.
My view is that there are things added to household or cooking salt which are very harmful to fish - so it is always a good idea to check the label for any additives.
Here's a list of agents that are really bad:
Magnesium: found in: cow block salt
YPS This is prussiate and may be worst.
Cyanuric acid (found in some BUT not all anticaking agents. That's why it's better to exclude all as its often not listed. But it does explain why some people have successfully used table salt while others have not)
Iodine: This will kill bbs but apparantly will not harm fish at low doses
Sodium aluminosilictae is safe and sometimes added to salt.