First Anthony, I do not think that most understand that Silica is very high in NSW, on the order of 2-3 ppm. Silica levels in reef tanks are usually well below that found on coral reefs.
The MBARI shows std NSW it at:
100 ?mol/L
100 ?mol/L x 28.0855 MW = ~ 2,800 ppb = 2.8 ppm avg NSW
At surface levels it can be as low as 1 ?mol/L or 0.001 ppm in some areas.
If an RO/DI shows ~ 0 TDS there is not much of anything there in Silica, despite rumors you may have heard.
The only algae that uses silica are Diatoms, others do not. Diatoms are usually only a problems in newly set up tanks and then disappear, as they are early pioneers in new systems.
A TDS can not tell what ions, be it Silica, PO4, Ca++, Mg++, CO3-- etc.., are there. All ions have either a positive or negative charge. These charges make the water conductive and that conductiveness can be measured via a conductivity meter, which measures the conductiveness between two probes. That conductiveness, of all those ions, can them be converted into a TDS reading. That reading can not tell us what ions or their concentration is. However, if we were testing a pure known ppm solution of NaCl we can. Lets pretend that solution is 100 ppm NaCl. Thus, the TDS meter will show 100 ppm TDS as NaCl and it is in fact 100 ppm NaCl. Now we have another solution of NaHCO3, KCl, MgSO4, etc, etc, and it reads 100 ppm using that meter. That does not mean there is 100 ppm of those ions, Na+, SO4-, Mg++, etc in the water. That 100 pm reading is just telling you that the conductiveness created by those ions is equal to the conductiveness of a pure NaCl solution of 100 ppm NaCl. Our second solution has NO NaCl in it, yet it gives a reading of 100 ppm as NaCl, as those ions are conductive. Any free ion has its own conductiveness. If it is not free ion, but a solid like CaCO3 or is a molecule there is no charge. Thus, no reading. CO2 is a molecule and has no conductiveness. Thus, a sample of water could have 0 TDS and still have 10 ppm CO2.
It tests for one silicate, siliconoxide which can be converted to Silicon by x 2.1. There is not such thing as 4 silicates. The kit does use 3 Si reagents. There are two forms of 'Silica" 
