Why silicate isn't an acute problem
Unlike ammonia or nitrite, silicate isn't toxic to fish or most plants — and there's no widely recognized "critical" threshold like there is for other readings. Silicate mainly matters because it's the nutrient diatoms use to build their cell walls.
The brownish, dusty film that often appears in the first few weeks after setting up a new tank is usually exactly that — diatoms, encouraged by available silicate combined with a microbial community that hasn't fully established yet to compete with them.
Where silicate comes from
New sand or substrate: Some substrates, especially natural sand, release silicate into the water during the first few weeks.
Tap water as a source: Silicate content in tap water varies noticeably by region and geology.
Certain rocks: Some decorative rocks can also release small amounts of silicate.
What to do about a diatom outbreak
The good news first: diatoms in newly set-up tanks are usually a temporary phenomenon that resolves on its own after a few weeks, once the tank's biological balance stabilizes — often another, usually more harmless algae type briefly takes over before everything settles down.
Be patient: Usually the most effective step in a new tank — just wait for it to cycle in.
Remove it manually: The film usually wipes off easily with an algae magnet or a soft cloth.
Silicate-absorbing filter media: Specialized resins bind silicate directly if the problem is persistent.
Use RO water: Noticeably reduces silicate input through top-off and change water.