Many water sources contain silicates or silicic acid, or compounds that contain these elements and these compounds eventually break down, adding silicates and silicic acid to your aquarium's water.
In aquariums where sandy or other forms of substrate are used, silicate can leach "out" of these compounds, and enter the tank's water very quickly.
diatoms can also "pull" small amounts of silicates directley from your glass and silicone sealant. your not likley going to find a 100% silicate free environment, But as I said before once they use up the soluble silica present in the system (since your useing Ro/di you wont be adding more) they should pass