The short answer
Brown algae is diatoms — a soft, dusty brown film that coats glass, substrate and plants, mostly in new tanks. The good news: it’s the least worrying algae there is. In a young tank it almost always fades on its own as the biology matures. Just wipe it off, keep the light modest, and stay consistent with water changes while the tank settles. In older tanks it’s less common but easily managed.
In a new tank: wipe and wait
If your tank is only a few weeks or months old, a brown diatom bloom is completely normal — it appears as the tank cycles and its chemistry stabilises.
- Wipe it off. It comes away easily with an algae scraper or a cloth, and syphons off the substrate during a water change.
- Keep the light modest. A 6-hour photoperiod on a timer while the tank is young avoids feeding other algae.
- Be patient. As the tank matures, diatoms typically run out of the silicates and unstable conditions they need, and fade within the first couple of months.
In an established tank
Persistent brown algae in a mature tank usually has a specific cause worth checking:
- Low light can favour diatoms — make sure your fixture and photoperiod suit your plants.
- Silicates in the tap water feed diatoms; if it’s a recurring battle, that can be the source.
- Nutrient build-up from overfeeding or skipped water changes gives them fuel — feed less and keep up weekly changes.
The bottom line
Brown algae is mostly a phase, not a problem — especially in a new tank, where patience and consistency are the whole answer. For related help see how to prevent algae in a new tank and our how to get rid of aquarium algae guide.