Brown Algae (Diatoms) in a New Tank: Why It Happens and When It Stops

A near-universal new tank phase

Diatoms are single-celled algae that thrive on silica, which is abundant in new sand, new glass, and fresh water sources, especially well water or certain tap supplies. Almost every new tank goes through a diatom bloom in the first 2–8 weeks.

Why it resolves on its own

As silica in the water column gets depleted and competing organisms (other algae, bacteria, eventually grazers) establish, diatoms naturally decline. This is a succession process, not a problem requiring aggressive intervention in most cases.

What actually helps

• Manual removal (wiping glass, light gravel vacuuming) reduces the visible bloom without addressing the cause — useful for appearance while you wait it out.
• Nerite snails and other grazers happily eat diatoms and can meaningfully reduce visible buildup.
• RO/DI water if your tap water is unusually high in silicates, cutting off the fuel source rather than just removing the result.

When it's not "just diatoms"

If a brown coating persists well past 8–10 weeks in an established tank, or returns repeatedly in a mature system, it may be silicate from a specific source (certain substrates, some water conditioners) rather than the normal new-tank phase — worth testing silicate directly if the timeline doesn't match the typical pattern.