Brown Algae (Diatoms) in a New Tank: Why It Appears and How to Clear It
The brown dusty film that coats a new tank in its first weeks, why it means the tank is still immature, and how to clear it without chemicals.
What diatoms look like
Diatoms, commonly called brown algae, show up as brown, dusty patches and thin coatings on the glass, substrate and leaves. They are one of the most common sights in the first weeks of a new setup; Tropica lists diatoms among the thin coating algae that settle on surfaces.
Why they show up
The 2Hr Aquarist is blunt about the cause: the presence of diatoms almost always means the tank is not yet biologically mature. They typically appear within the first 1-3 weeks of setting up. Rather than a specific mistake you made, they are a symptom of an immature system that has not yet built up its microbial community.
How to clear it
- Be patient — the 2Hr Aquarist notes diatoms usually persist for about 2-6 weeks and then fade on their own as the tank matures, with no special treatment.
- Wipe the glass and siphon the worst off surfaces during your water changes.
- Add diatom grazers early: Otocinclus, nerite snails and shrimp. Tropica recommends introducing algae-eating snails and shrimp soon after setup.
- Keep up frequent water changes in the first weeks — Tropica suggests 25-50% a couple of times a week early on.
Sources: The 2Hr Aquarist, How to get rid of brown algae (diatoms) and Algae and tank maturity (www.2hraquarist.com www.2hraquarist.com); Tropica Plant Guide, Types of algae (tropica.com) and Starting a new aquarium (tropica.com).