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Brown algae is one of the first things many new aquarium owners see. A dusty brown coating appears on the glass, sand, rocks and plant leaves, often within a few weeks of setting up. It looks alarming, but it's normal, usually harmless and normally goes away on its own.
What is brown algae?
Brown algae in aquariums is usually diatoms, single-celled algae with shells made of silica. It forms a soft brown film that wipes away easily. In marine tanks it's common in the first few months too.
What causes brown algae?
- A new tank: diatoms thrive while the tank is still settling and before other organisms take hold.
- Silicates: from tap water, some sands and new substrates. Diatoms use silica to build their shells.
- Excess nutrients such as nitrate and phosphate.
- Low light: diatoms cope with dim light better than green algae.
Is brown algae harmful?
No. Diatoms aren't harmful to fish. A thick coating can block light from plant leaves, so it's worth removing.
How to get rid of brown algae
- Be patient. In most new tanks, diatoms disappear within 2–8 weeks as the tank matures.
- Wipe it away: use an algae pad or magnetic cleaner on the glass. See algae pads and scrapers. Gently brush plant leaves and stir the top layer of sand.
- Siphon it out during water changes with a gravel cleaner.
- Use RO water if your tap water is high in silicate. See our reverse osmosis units and guide to RO water.
- Keep nutrients down: feed carefully and keep up weekly water changes. Test with a test kit.
- Grow more plants to compete for nutrients. See our easy plants guide.
What eats brown algae?
Several clean-up crew species graze diatoms eagerly:
- Otocinclus catfish: the best fish for diatoms, once the tank is a few months old.
- Nerite snails such as the zebra nerite and red onion snail. They can't breed in freshwater, so they won't overrun the tank.
- Bristlenose plecos and shrimp.
Read more in our snails guide, new tank syndrome and our algae control guide.
