Salt levels

Thanks for your help / info on goldfish and salt. One reason I am
interested
apart from the need to care for our fish is that we were worried that
the addition of salt to the rainwater might be masking any toxins
present in the water supply. We put our first fish in a tank with aged
aerated straight rainwater and it appeared to be o.k. but died
overnight. So we cleaned the tank out, added new water checked p.H
and
bought a cheap fish put it in and it died 2 hours later. Next we added
salt (rainwater conditioner) to the water and hey presto problem
solved
, no more dead fish. But I still have a nagging doubt that the salt
may
be masking a toxic residue problem (perhaps from contamination from
agricultural sources). So I was wondering if goldfish can gradually be
acclimatized to a low salt level. If they'll survive without the salt
then I'll know that the water is O.K. In addition to using the water
for our fish we rely on it for all our household use and would like to
know it is safe.
Although it is no doubt best in the long term for goldfish to have
some salt in the water is it possible to gradually reduce the level
until they're in almost saltfree water? If they'll survive in this
I'll feel confidant there is no other serious contamination. Thank you
again.