The old books that tell you to only use kosher salt or aquarium salt date
from
the days when arsenic was used to keep table salt free-flowing. They
don't put
arsenic in table salt any more, so it's safe to use in fishtanks.

they used to put arsenic in table salt!?!? Yowza!!! I know arsenic can be
expelled from the body, but still.. what human kind used to do to
themselves... (then again, Coca-Cola used to contain trace amounts of
cocaine until they switched it out for caffeine instead... so.. go figure
http://www.snopes.com/cokelore/cocaine.asp)

linda