Brown algae in a fish tank (diatoms): causes and fixes
Symptom: A brown, dusty film on the glass, sand, plants or rock that wipes off easily.
Likely causes, ranked
1. A new tank
very commonDiagnosticThe tank is weeks old. A brown film appears on glass, sand and decorations, often right after the nitrogen cycle finishes.
FixWait it out and wipe the glass. sera describes diatoms as pioneer organisms in newly set-up aquariums that usually decline after a short while. Don't chase it with chemicals.
2. Silicate in the tap water
commonDiagnosticDiatoms keep coming back in an established tank, especially right after water changes with tap water.
Fixsera says diatoms need silicate to build their shells and that drinking water isn't treated to remove it. If nothing else works, use RO or deionised water for top-ups and water changes. For freshwater fish, remineralise RO water to the hardness they need before use.
3. Excess nitrate and phosphate
commonDiagnosticHeavy feeding, a full stocking list, nitrate well above 10 mg/L.
Fixsera suggests fewer fish or less food, fast-growing plants and large water changes with low-nitrate, low-phosphate water, aiming for nitrate around 5–10 mg/L and phosphate around 0.1 mg/L without letting either reach zero. Expect several weeks before you see a change.
4. Plants not growing well
less commonDiagnosticA planted tank with slow, stalled growth and brown coating on older leaves.
Fixsera recommends good lighting and, if needed, CO2 and a suitable substrate so the plants outcompete the algae.
Common myths
ClaimBrown algae means the tank is dirty.
RealityIn a new tank it's a normal stage that usually passes on its own (sera).
ClaimYou need an algaecide.
RealityDiatoms fade with time in new tanks and respond to silicate and nutrient control in old ones; sera doesn't suggest chemicals.
Questions
Is brown algae bad for fish?
It isn't toxic; sera notes that the silicate diatoms use isn't toxic either. It's an eyesore that usually fades in new tanks.
How long do diatoms last in a new tank?
Usually a short while; sera describes them as pioneer organisms that decline on their own. In established tanks, changes take several weeks to show.
What eats brown algae?
sera says some fish and snails readily eat diatoms. On reefcalcs, see the trochus and turbo snail sheets for reef grazers.
Why do I get brown algae after water changes?
Probably silicate in the tap water; sera suggests RO or deionised water if it persists.
Related
Sources & references
- 1.sera — Brown layers on plant leaves: diatoms (manufacturer FAQ)https://www.sera.de/en/service/faq/faq-detail/brown-layers-on-plant-leaves-diatoms
Last reviewed