Tap water, purified water, mineral water, spring water, etc. have too many impurities in them and will not produce Colloidal Silver good enough for normal use.
Steam Distilled water or Medical Grade pure water is the only water that is normally pure enough to work correctly.
Some people use Reverse Osmosis water and say it works well, but it totally depends on the quality of your RO filter and how much it has been used. It may work, it may not. You'll know by the finished product.
When you make Colloidal Silver it should be clear or have only a slight yellow tint. If it is cloudy, milky looking, or has a dark color, don't use it.