I know for a fact that "scaleless" fishes like cories do not do well with the salt treatment.
I also wouldn't do it with any fish that prefer extremely soft water.
I'm the nicest person on the PLANET.
(wanted to keep the images in, so I uploaded it to my web site.) http://aquastock.webs.com/fishdiseases.html
Some other diseases that come to mind (assuming freshwater only) are:
Mouth/Fin Fungus (cottonmouth)
Grayish white marks or patches on the body of the fish or around its mouth are the first obvious signs of Columnaris. These patches appear like threads, especially in the vicinity of the mouth. Hence, this is often confused with another disease called "Mouth Fungus". The fins of the fish may show signs of deterioration, the gills get affected and slowly sores start appearing on the body. During a later stage the fish can hold its fins very close to the body and not spreading them. Columnaris disease is caused by bacteria, and is mostly a direct result of poor water quality. Sometimes, the shock of being introduced to the new aquarium leads to Columnaris disease in newly imported fish. Anti-bacterial medications will be enough if the disease is discovered and treated at its early stages. The disease will however invade the internal organs during the later stages and antibiotics are then required to cure it. )
Just so you all know, this information took me about 2 minutes. I was going to keep going, but I suddenly realised how easy it was. I prefer to help those who help themselves. I will probably revoke my link soon since I have limited bandwidth. Please only click my link if you really feel you can help (ie you have kept fish for OVER 10-20 years, or are a moderator.)
His link was clean, so feel free to use it. You have to download it, but just click 'open' instead of 'save'