If you use a relatively low-mineral, low-TDS spring water or tap water and filter it through Berkey, and you get close to 0 TDS, AND you can make Colloidal Silver without turbidity, you're golden. I have found colloidal gold to be a little more finicky than Colloidal Silver when it comes to contaminants, so your mileage may vary.
Quoting from the Berkey homepage: "The revolutionary Black Berkey® purifier elements are so powerful that they can remove food coloring from water without removing any of the beneficial minerals your body needs." Since minerals are exactly what gives us trouble in colloid production AND what shows up on a TDS reading, I'm surprised your TDS meter shows Berkey water as being close to 0.
Only you can tell us if your product is good, and therefore, if the water is pure enough. Also, making a higher PPM product will more easily demonstrate turbidity. At 20 PPM it's hard to distinguish a perfect product from a "passable" one. But at 50 and higher PPM this gets progressively easier.