Brown Diatoms in Your Tank? Here Is Exactly What to Do

You set up your beautiful new planted tank, and within a week or two, everything is covered in a slimy brown film. The glass, the plants, the hardscape, all of it looks like someone dusted it with cocoa powder. Congratulations, you have diatoms. And before you panic, here is the most important thing to know: diatoms are completely normal in new tanks and they go away on their own.

What Are Diatoms?

Diatoms are single-celled algae with silicon-based cell walls (frustules). They are not the same as green algae, hair algae, or cyanobacteria, they are their own category. The brown color comes from the pigment fucoxanthin, which masks the green chlorophyll underneath. Diatoms appear as a soft, easily-wiped brown coating on every surface in the tank.

The Timeline

• Week 1-2: First brown patches appear, usually on glass and slow-growing plant leaves
• Week 2-4: Diatoms spread across most surfaces. This is the peak, it looks terrible but is temporary.
• Week 4-8: Diatoms begin receding as silicates deplete and other organisms (beneficial bacteria, green algae) outcompete them.
• Week 8-12: Diatoms mostly gone. Occasional patches may appear but do not persist.

What to Do (and What Not to Do)

Do

• Wipe the glass: Use an algae scraper or magnetic cleaner every 2 to 3 days. Diatoms come off easily, they are not attached firmly.
• Add a cleanup crew: Nerite snails, otocinclus catfish, and amano shrimp devour diatoms. A crew of 3 nerite snails can keep a 20-gallon tank's glass clean through the entire diatom phase.
• Maintain water changes: Regular 20 to 30 percent weekly water changes dilute silicates and maintain water quality.
• Be patient: This is the most important action. Diatoms resolve themselves. Every new tank goes through this.

Do Not

• Reduce lighting: Diatoms thrive in low light and are outcompeted by plants in higher light. Reducing light helps diatoms, not you.
• Use chemical algae treatments: They are unnecessary and can harm your developing beneficial bacteria and plants.
• Tear down the tank: The diatom phase resets if you start over. You would go through it again.
• Obsessively clean every surface: Focus on the glass (viewing surface) and leave the rest. Diatoms on hardscape and substrate are being consumed by the developing biofilm ecosystem.

When Diatoms Are NOT Normal

If brown algae appears in an established tank (6+ months old), something has changed:

• New substrate or hardscape: Adding new materials releases fresh silicates. Diatoms will appear temporarily.
• Water source change: If your utility switched water sources, silicate levels may have increased.
• Reduced light or photoperiod: If you recently reduced lighting, diatoms may have gained a competitive advantage over plants and green algae.
• Filter disruption: Major filter cleaning or replacement reduces the bacterial population that competes with diatoms for nutrients.

Diatom-Eating Cleanup Crew

Make sure your tank is properly cycled before adding a cleanup crew, see our cycling crash course. And check stocking levels for your cleanup crew with our tank size calculator.