You can do it with no dissolved solids added i.e. salt, it just takes longer. The amount of soda I'm talking about is only the bit one could dip with the tip of a tooth pic, VERY little. 50 volts will get it going a little better because NO water is absolutely pure. However, 50 VDC will start to run away at some point as the water becomes more and more conductive with the silver particles so close monitoring is necessary. This is also true of lower voltages but not as critical.
Colloidal Silver is a miracle and a lifesaver