So just to confirm, 0 TDS should not have any phosphate, nitrate or silicate in it?
It doesn't mean that, exactly. It means you will not have "too much" of anything, including those. If your test is good enough, you may see them, but if the amounts added are low (say, 1/100th of the amount added in foods in the same time period), it is insignificant.
For phosphate, I discuss that here:
Aquarium Chemistry: Phosphate And Math: Yes You Need To Understand Both
https://www.advancedaquarist.com/2012/3/chemistry
from it;
Comparison of Food Sources of Phosphate to Other Sources
What about other sources of phosphate, like the "crappy" RO/DI water containing 0.05 ppm phosphate? A similar analysis will show it equally unimportant relative to foods.
Let's assume that the aquarist in question adds 1% of the total tank volume each day with RO/DI to replace evaporation. Simple math shows that the 0.05 ppm in the RO/DI becomes 0.0005 ppm added each day to the phosphate concentration in the aquarium. That dilution step is critical, taking a scary number like 0.05 ppm down to an almost meaningless 0.0005 ppm daily addition. Since that 0.0005 ppm is 40-600 times lower than the amount added each day in foods (Table 4), it does not seem worthy of the angst many aquarists put on such measurements. That said,
tap water could have as much as 5 ppm phosphate, and that value could then become a dominating source of phosphate and would be quite problematic. Purifying tap water is important for this and many other reasons.
