Brown algae is not actually algae—it's a diatom that thrives in new tanks and high-silicate water

Brown algae, also called diatoms, looks like a brown dust or film coating your glass, decorations, and substrate. Despite the name, it is not a true algae but a single-celled organism with a glass-like shell. It appears most often in newly established tanks (the first four to eight weeks) because diatoms feed on excess nutrients and silicates that accumulate before your tank's nitrogen cycle stabilizes.

The good news is that brown algae is harmless to fish and plants, and it almost always disappears on its own once your tank matures. You can speed up the process by removing it manually, improving water conditions, and introducing organisms that eat it. Unlike green algae blooms, brown algae does not indicate a serious problem—it is a normal part of tank cycling.

Key Takeaways

  • Brown algae thrives in new tanks and water with high silicate levels, and it typically disappears within a few weeks as your tank stabilizes.
  • Manual removal with a soft brush or cloth works when ready but does not stop it from returning until water conditions improve.
  • Increasing water changes, reducing light hours to eight per day, and adding a powerhead to improve water flow all reduce the conditions diatoms need to grow.
  • Algae-eating fish and invertebrates like otocinclus catfish, amano shrimp, and nerite snails consume brown algae but work best alongside improved water conditions.
  • Tap water high in silicates can fuel brown algae growth; using reverse osmosis water or a silicate-removing resin helps if the problem persists after four weeks.

Manual removal: the fastest visible fix

The quickest way to clear brown algae from glass is to wipe it off with a soft cloth or algae scraper. For glass panes, use a plastic or magnetic algae scraper—avoid metal scrapers, which can scratch the glass. For decorations and plants, a soft-bristled toothbrush or dedicated aquarium brush works without damaging delicate surfaces.

Manual removal gives you when ready results, but the diatoms will return within days if the underlying conditions do not change. Think of it as cleaning a symptom rather than treating the cause. It is worth doing once or twice to improve visibility while you work on the water conditions that actually stop brown algae from growing back.

Water changes and flow: the foundation of control

Brown algae feeds on excess nutrients and silicates that build up in new tanks. Perform a 25 to 30 percent water change twice per week for the first month. This removes accumulated waste and silicates faster than your filter can process them, which starves the diatoms of fuel.

Poor water flow also favors brown algae. Add a small powerhead or adjust your filter outlet to create gentle current throughout the tank. Diatoms settle and thrive in dead spots where water barely moves. A powerhead costs $15 to $40 and moves water without adding heat or noise.

Reduce your light cycle to eight hours per day during the first month. Brown algae grows slower in lower light. Once your tank is four to six weeks old and the diatoms have cleared, you can return to a normal ten to twelve hour photoperiod.

Algae-eating fish and invertebrates

Several aquarium animals consume brown algae directly. Otocinclus catfish are small (one to two inches) and eat diatoms off glass and plants without harming either. They need to be kept in groups of at least four and prefer established tanks with stable water. Amano shrimp are larger and more aggressive algae eaters; they work in community tanks but may prey on very small fish fry. Nerite snails are slow but thorough and do not reproduce in freshwater, so they will not overpopulate your tank.

These animals work best when combined with improved water conditions. Introducing them to a tank with poor flow and high nutrients will not solve the problem on their own—they are a supporting step, not a replacement for water changes and light adjustment. Wait until your tank is at least two weeks old before adding any new inhabitants, as they need stable conditions to survive.

Silicate levels in tap water

If brown algae persists beyond four weeks despite water changes and reduced light, your tap water may contain high levels of silicates. Silicates are minerals that enter water through sand and rock and are not removed by standard aquarium filters. Some municipal water supplies have higher silicate content than others; this varies by region and water source.

If you suspect silicate is the problem, you can test your tap water using a silicate test kit (available online for $15 to $25). If levels are above 1 mg/L, consider using reverse osmosis (RO) water for water changes, or install a silicate-removing resin cartridge on your tap line. RO water is available at many aquarium stores in bulk, or you can purchase a small RO unit for $100 to $200 if brown algae is a recurring issue in your area.

When to stop worrying and let time work

In most cases, brown algae disappears completely between weeks four and eight of a tank's life, even without intervention. Your tank's nitrogen cycle stabilizes, beneficial bacteria colonize the filter, and nutrient levels drop naturally. Once this happens, the conditions that favor diatoms no longer exist, and they vanish.

If you have performed water changes, adjusted light, and added flow, you have done everything that speeds up this process. At that point, patience is the final step. Brown algae is not a sign of failure or a tank emergency—it is a normal part of cycling, and it will resolve.

Frequently Asked Questions

Is brown algae harmful to fish or plants?

No. Brown algae does not produce toxins and does not compete with plants for nutrients the way green algae does. Fish ignore it. The only downside is appearance—it makes the tank look dirty even though the water is fine.

Can I use algae-killing chemicals to remove brown algae?

Algaecides are not necessary and can harm beneficial bacteria in a new tank. Since brown algae resolves on its own within a few weeks, chemicals add risk without benefit. Stick to water changes, light reduction, and manual removal.

Why does brown algae come back after I clean it?

Cleaning removes the visible diatoms but not the spores or the conditions that allow them to grow. Brown algae returns quickly if silicates and excess nutrients are still present. Improve water flow, reduce light, and perform frequent water changes to address the root cause.

Do I need to do anything special if I have live plants?

Live plants actually help reduce brown algae by consuming excess nutrients. Keep your plants healthy with regular fertilizer and adequate light, and they will compete with diatoms for resources. Otocinclus catfish are safe around plants and are a good choice if you have a planted tank.

How long does it take for brown algae to go away completely?

In a new tank with no intervention, brown algae typically peaks around week two and fades by week six or eight. With water changes, reduced light, and improved flow, it can clear in three to four weeks. The exact timeline depends on your tap water quality and tank size.