Hm, salt and water quality? The salt has nothing to do with water quality. Water quality is how well the tank is filtered, oxygenated and maintained by routine water changes. The affect of the above mentioned parameters, is related to the Nitrogen cycle, how much Ammonia, Nitrites, Nitrates..etc is accumulating in a closed system such as an aquarium.
Salt on the other hand is a mineral. In a natural environment, salt is obviously natural part of the soil and therefore runs off into the water system. In a tank system there is no natural source of salt, therefore it has to be added. Salt then is added not for the water quality, actually it is added for the fish. Certain fish require salt, example would be brackish species (archers, mollies, gobies..etc). It is also added as a preventative treatment for scaleless fish agaisnt Ich (loaches, catfish, eels..etc). Reason is some medicinal treatment are harmful to scaleless fish, therefore salt is used instead.
As for Piranha, salt isn't really needed since they are so hardy. You could add some as a mineral for the fish but it does not improve the water quality.