Brown Algae (Diatoms) in Aquariums

Brown algae are diatoms, single-celled algae with silica cell walls. They bloom in new tanks fed by silicate and usually fade as the system matures.

Identification

Brown algae in aquariums are diatoms, single-celled algae with a silica cell wall called a frustule. Their yellowish-brown colour comes from chloroplasts containing the pigment fucoxanthin. On glass, substrate and plants they form a dusty, flour-like brown coating that can usually be wiped away easily.

Causes

Diatoms build their cell walls from silica and require dissolved silicic acid to grow, which is why they are tied to silicate levels in the water. They commonly appear as a bloom in new tanks where silicates and phosphates are still high and biological balance has not yet established. High phosphate is also implicated.

How to control it

• Wipe diatoms off the glass with an algae scraper; they detach easily.
• Add grazers such as otocinclus catfish, nerite snails and shrimp, which readily eat diatoms.
• Perform water changes to dilute nutrients.
• Allow growing plants to draw down excess phosphate over time.

When it passes

In most new aquariums a diatom outbreak is temporary. As the tank matures, available silicate and other nutrients are consumed and biological filtration stabilises, and the brown coating typically fades over a few weeks without long-term treatment.

Prevention

• Be patient during the first weeks of a new setup.
• Avoid overfeeding so phosphate stays low.
• Keep up regular water changes and cleaning.
• Maintain healthy plant growth.