Brown Algae (Diatoms) in Aquariums

Of every algae problem in the hobby, brown algae (diatoms) is the one I most enjoy seeing. It looks alarming — brown slime coating the glass, the substrate, even plant leaves — but it's almost always a new-tank phase that resolves itself in 2–6 weeks. I tell new aquarists: if your tank is under two months old and growing brown dust, you're fine. Wipe it off, do a water change, and wait.

The trick is knowing when brown algae is just a phase and when it's pointing at a real problem. Let's break down how to tell the difference and what to do in each case.

Symptom Identification

Diatoms are easy to identify once you know what you're looking at:

• Brown, dusty coating on glass, substrate, decorations, and plant leaves
• Wipes off easily with a finger or magnet cleaner — comes off in sheets, not patches
• Returns within 24–48 hours after wiping in active phase
• Most common in tanks <8 weeks old, but can appear in mature tanks with silicate-rich water
• Coats everything evenly — not fuzzy like hair algae or slimy like cyanobacteria

The diagnostic test is simple: wipe a patch with your finger. If it comes off as a brown smudge that looks like fine dirt or rust, it's diatoms. If it's rubbery and pulls in strands, that's hair algae. If it sheets off in slimy greenish-blue patches that smell earthy, that's cyanobacteria (different fix entirely).

Cause Diagnosis

Diatoms are single-celled organisms that build shells out of silica. They thrive when three conditions line up:

1. Silicates available in the water — from tap water, fresh substrate (especially sand and aragonite), new glass, or some sea salt mixes.
2. Low or shifting light spectrum — diatoms outcompete green algae at the yellow/red end of the spectrum, which is what older bulbs or weak stock lights produce.
3. New tank conditions — the biofilm and green algae that normally outcompete diatoms haven't established yet.

In a new tank (1–6 weeks), all three conditions exist simultaneously. That's why diatoms are the universal new-tank algae. As the tank matures, green algae establish on surfaces, silicates get used up, and diatoms fade. This is the "wipe and wait" case.

In an established tank that suddenly grows diatoms, the cause is usually one of: tap water high in silicates, an old bulb whose spectrum has shifted, or a recent biofilter disruption (medication, large water change with untreated water). These need active intervention.

Treatment Protocol

For New Tanks (Under 8 Weeks Old): Wipe and Wait

1. 1
   Wipe the glass during water changes

   Use an algae magnet or sponge. The brown film comes off easily. The water change removes the dislodged diatoms instead of letting them resettle.

2. 2
   Vacuum the substrate surface

   Diatoms settle on top of gravel and sand. Light surface vacuuming during water changes removes them.

3. 3
   Increase lighting slightly

   If you're running low light, bump up to 6–7 hours per day. Stronger light favours green algae over diatoms. Don't go over 8 hours or you'll trade diatoms for green water.

4. 4
   Be patient

   Diatoms in a new tank fade on their own as silicates deplete and green algae establish. This typically takes 2–6 weeks. Resist the urge to add chemicals.

For Established Tanks: Find the Trigger

1. 1
   Test silicates in tap water

   If your tap reads above 2 ppm silicate, that's your source. Use RO water for changes, or run a phosphate/silicate remover pad in the filter.

2. 2
   Replace old light bulbs

   Bulbs older than 9–12 months shift toward the yellow/red spectrum diatoms love. Swap in new full-spectrum bulbs.

3. 3
   Add otocinclus or nerite snails

   Both are diatom specialists. A group of 4–6 otos in a 20-gallon tank will clear diatoms in days. Make sure your tank is cycled and stable before adding them — otos are sensitive to water quality.

4. 4
   Wipe and water change weekly

   Active removal plus the underlying fix clears the outbreak within 2–3 weeks.

Prevention

Once a tank matures past the diatom phase, prevention is largely automatic. A few habits help:

• Don't add otocinclus to brand-new tanks — wait until the tank has been cycled and stable for at least 4–6 weeks. Otos need established biofilm.
• Replace light bulbs annually — even LEDs shift spectrum over years; fluorescents shift faster.
• Know your tap water — if silicates are high, plan for RO or chemical removal from the start.
• Maintain a regular water change schedule — 25–30% weekly prevents silicate accumulation.
• Keep nerite snails in any tank 10 gallons+ — they're excellent diatom grazers and stay small.

Common Mistakes to Avoid

Diatoms are the most over-treated algae in the hobby. Avoid these common errors:

• Treating a 2-week-old tank like it has a problem — brown algae in tanks under 6 weeks old is normal and self-limiting. Wiping + waiting is the entire fix.
• Adding otocinclus to a brand-new tank — otos need established biofilm and stable water. Adding them to a 2-week-old tank usually results in starved otos within a week.
• Buying one oto instead of a group — otos are schooling fish that stress and starve alone. Always get at least 4–6, and never just one "to clean the algae."
• Using algaecide on diatoms — diatoms clear on their own once silicates deplete. Algaecides kill them faster but leave dead cells to decompose and spike ammonia — solving nothing.
• Wiping the glass and not changing the water — the dislodged diatoms resettle within hours. Always wipe during a water change so the siphon removes them.
• Reducing light to "kill" diatoms — counter-intuitively, more light favours green algae over diatoms. Reducing light can actually extend the diatom phase.
• Assuming brown dust is always diatoms — cyanobacteria (blue-green algae) can look brown and slimy. If it smells bad and sheets off in patches, that's cyano, not diatoms — different protocol entirely.

The single most useful mindset shift with brown algae: in a new tank, it's a phase, not a problem. Don't fight the phase — let it run its course with weekly wiping and water changes. In an established tank, find the silicate source instead of treating the symptom.

Quick Diagnosis Table

Frequently Asked Questions