Always use Distilled Water when you are making Colloidal Silver.. Distilled water has a very low conductivity and allows for only a very small current to flow between the 2 silver wires which produces the ultra fine silver particles. Silver nano particles mimic the way silver ions exist naturally in plants and spring water!

[link to www.silver-colloids.com]

ELECTROLYSIS :
When electric current passes through silver, some silver atoms at the interface with the water will loose an electron changing the atom into an ion.