The short answer
Brown “algae” is almost always diatoms — microscopic organisms that coat glass, substrate and plant leaves in a soft, dusty brown film that wipes away easily. It’s one of the most common things to appear in a new tank, usually in the first few weeks, and in most cases it fades on its own as the tank matures.
Why it appears
Diatoms thrive on silicates and other nutrients that are abundant in a freshly set-up aquarium — from new substrate, tap water, and a biological filter that hasn’t fully matured. Low or unstable light in a young tank suits them too. Because these conditions are temporary, the brown film normally peaks and then recedes as beneficial bacteria establish and silicate levels drop.
Common triggers include a brand-new setup, a recent substrate change, or new water high in silicates.
How to clear it faster
- Wipe it off: it comes off glass with a pad or magnet cleaner very easily — see how to get algae off aquarium glass.
- Add cleanup crew: Otocinclus catfish and nerite snails graze diatoms enthusiastically and will keep leaves clean.
- Stay consistent: regular water changes export excess nutrients while the tank matures.
- Stabilise light: a steady 6–8 hour photoperiod helps plants outcompete algae. See our aquarium lighting hub.
If it won’t go away
If brown algae persists well beyond the new-tank phase, it usually means ongoing excess nutrients or weak, old lighting. Growing more healthy plants is the best long-term defence — start with our easiest beginner plants. For a broader plan against every algae type, read how to get rid of aquarium algae.