Interesting you raise this. I thought the practice of adding salt to freshwater aquariums for its "magical" properties had gone the way of the dodo decades ago. It is as outdated, and scientifically disproven, as the oldtimer's belief that water changes are bad for aquarium fish. That said, I actually heard a LFS employee last weekend telling a lady and her kids to add salt to their tank. I guess this LFS employee's reference book was his trusty 1935 edition of Innes.
There are medicinal uses for salt. A salt dip for example, is a routine treatment for wild caught fishes to remove parasites. It can also be helpful when transporting fish or treating catfish for brown blood disease. However, unless you are keeping brackish or euryhaline fishes there is no reason to add salt to an aquarium. Ostariophysi (catfishes, characins, cyprinids and gymnotiformes) are particularly susceptible to salt even in very low concentrations. Even Innes noted that Corydoras died very quickly when salt was added to their tank. Perciformes (cichlids) tend to tolerate higher salt levels since they had marine ancestors.
Don't count on old aquarium literature on this one. Plenty of studies have been done by actual scientists for the aquaculture industry.
-Shane
