I don't mean to drag up old bones, but this is thread brings up some important issues.
Marine salt is more than simply Sodium Chloride. It contains buffers and other additives that simulate marine conditions, and (with most good brands) is tested in labs first before a company puts it on the shelves for our use.
Aquarium Salt is an unregulated product that is really no more than Sodium Chloride, or table salt, although you never really know what you're getting with aquarium salt. There also is no scinetific evidence that shows that Aquarium salt is useful for
anything in our aquariums, so I would rather not risk it.
However, I suppose if you like your fish nicely seasoned then this is the way to go!
There is more to marine conditions than just salting up the water, there are several other things such as pH and kH that are controlled and stabilized by marine mixes that ensure a healthy and stable environment for the fish.
I won't get into my opinions on Aquarium Salt's use as a tonic... but anyways, on to the most important thing: the fish!
Fresh water fish are typically able to with stand small amounts of salt. However, it is not good for them (if it was, they'd be brackish fish!) and certain kinds of fish are extremely sensitive to it (including many catfish such as plecos and corys).
Brackish water fish, on the other hand, can often tolerate fresh water conditions for short periods of time. If this weren't true then we would never see live brackish water fish because 99% of the time they are put into freshwater tanks at fish stores.
However, brackish water fish will not live for long in freshwater even if it isn't immediately apparent.
It is always best to dedicate your tank to species that enjoy the same water conditions including salt, temperature, and tank size.