Until recently I had never seen any evidence that there was anyone who disputed the usefulness of salt in the freshwater aquarium.  The reason I would believe it are clear.  Everything I have read about the issue says that salt is safe for short term treatment of disease in many freshwater fish (not so much in some sensitive fish, depending on the amount).  Then one day I replied a suggestion to use salt to treat a problem someone was having with a fish.  I was quickly told why salt should never be used in an freshwater aquarium.  I paid no attention at first.  That is when I started research.

    1/2 of aquarium experts say salt is good, 1/8 say salt is bad, 1/2 say use salt sometimes, but not other times, and 1/2 have mixed opinions.  The point is, there is no right or wrong answer.  Some fish prefer salt, others die in its presence.  The normal dose for users is 1 tablespoon per 5 or 10 gallons.  It can be aquarium salt ($1.30 per lb.) or pickling salt ($.30/lb.).  You can get 4 pounds of pickling salt cheaper than 1 pound of aquarium salt, so I get pickling salt.  Kosher salt can also be used. 

    A controversial salt is iodized table salt.  Many people go mad when somebody suggests using table salt.  "The anti-caking agents destroy the gills," they say, "or the iodine is poison to the fish."  Both have been shown to be false.  Table salt is probably no more dangerous than any other salt, and is much safer than bottled ich medications which can cause gill burns if over-dosed.

    I only use it for disease treatment because it is not really needed all the time.  Some say it helps keep fish healthy during the early stages of the cycle when ammonia and nitrites starts rising.  That is probably true, and I suggest its use for treating such problems if you prefer a traditional cycle to the new fish-less cycle.  The salt helps gill function and to prevent damage from nitrites.

    Salt can be used as a dip for external parasites and injuries.  Add 2-4 tablespoons per gallon and move the fish to that water for 30 seconds to a minute.  This is an old method of treatment for problems like bad cases of ich, velvet, and "fungal" infections.  If at any time the fish begins to roll over or gasp, it should be returned to the tank quickly.

    Salt is a good thing to have for some mollies, but they can do without it.  Most mollies are probably born in freshwater anyway.  You might want to ask your store if they had salt in their tanks at the store.  Still, salt might not be good for regular use for the tetras and catfish.  But there is a small amount of salt in every body of water.

    In conclusion, I do not recommend salt for regular use in freshwater tanks.  I do not believe it would hurt the fish, but I do not think it will help them enough to worry about it and spend the money.  For any disease, I do recommend using 1 teaspoon per 5 gallons.  For every "expert" who does not like salt, there are 7 "experienced fish keepers" who do like salt for some purpose in freshwater tanks.  I am not an expert (I consider that an insult), so I just listen to those who are experienced.

UPDATE: After a decade of experience with freshwater aquariums, I still do not use salt in my aquarium. (8-31-11)