the salt will help clean the water. Unless you are changing water minnies have been swimming in their own waste for a while. It can build up to lethal levels. I have well water(not run thru softener) and they seem to do good in that. Not sure about city water. Also I keep them just above freezing. Just my 2 bits on keeping minnies for a long time.
Salt has nothing to do with cleaning the water minnows need a certain amount of salt in there metabolism as you continue to change the water it depleats and they die. City water will kill them quick soft water is better than raw well water