Guide
Brown algae in aquarium
Diatoms on glass and decor: common in new tanks and how to manage.
- What it is: Brown “algae” is usually diatoms — single-celled organisms that thrive on silica and available nutrients.
- Why now: Very common in tanks under six months old; often fades as silicates are used up and the tank matures.
- First steps: Gently wipe glass and decor, stay on top of water changes, avoid overfeeding, and ensure filtration is adequate.
- Patience: In many setups diatoms recede on their own once the ecosystem stabilises — chasing chemicals usually is not necessary.
What are diatoms?
Diatoms are microscopic algae with silica shells. They form the dusty brown film you see on glass, plants, and hardscape — especially in newer setups. They are not the same as green hair algae or blue-green cyanobacteria, so the fixes differ.
Why brown algae shows up
New substrate and tap water can supply silicates. Combined with light and nutrients (fish waste, uneaten food), diatoms bloom quickly. Low plant mass (few plants to outcompete for nutrients) and immature biofiltration also make tanks more susceptible early on.
What to do
Manual removal: Use an algae pad or soft sponge on glass; gently rub leaves and decor. Water changes: Regular partial changes export nutrients and free-floating cells. Filtration: Clean mechanical media so it does not clog; maintain good flow. Light: Run a consistent photoperiod (often 7–9 hours) and avoid direct sunlight on the tank.
When to add helpers
Many small plecos, snails, and shrimp will graze lightly, but they will not “cure” a diatom bloom if the underlying causes remain. Treat them as a cleanup aid, not the main fix.
When to stop worrying
If parameters are stable (0 ammonia, 0 nitrite, controlled nitrate) and the tank is maturing, diatoms often thin out over several weeks. If brown coating persists for months with no improvement, test silicate if your kit allows, review feeding, and see our algae overview for comparison with other types.
Log nitrate, nitrite, and your maintenance schedule with App-aquatic.
Get the free appIs brown algae harmful to fish?
Usually not directly — it is unsightly. Very heavy growth can reduce light to plants and hint at excess nutrients, but the main issue is aesthetics and diagnosing tank maturity.
Will it go away on its own?
Often yes in new tanks as silicates decline and plants establish. Combine patience with consistent maintenance and manual removal.
Should I use chemicals to kill it?
Start with manual removal, light control, and water changes. Broad “algalicide” products can stress fish and plants if misused; fix the cause first.
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