Brown algae is usually a sign of excess nutrients and poor water flow, not a disease

Brown algae — also called diatoms — appears as a thin, dusty brown coating on glass, decorations, and substrate. It is not toxic to fish and does not indicate a serious problem, but it clouds the tank and suggests that water conditions need adjustment. Brown algae thrives when nitrate and silicate levels are high, water circulation is weak, or the tank is new and still cycling nutrients. The good news is that brown algae responds quickly to changes in lighting, water flow, and maintenance.

Unlike green algae blooms that turn water cloudy, brown algae stays localized and is easier to control once you address what is feeding it. Most aquariums experience brown algae at some point, especially in the first month after setup or after a period of neglected water changes.

Key Takeaways

  • Brown algae grows fastest in tanks with high nitrate levels and weak water circulation, so test your water and increase flow from your filter or add an air stone.
  • Reduce lighting to 6 to 8 hours per day — brown algae needs light to grow, and many tanks are lit too long anyway.
  • Do a 25 to 30 percent water change weekly and vacuum the substrate to remove decaying plant matter and fish waste that feed algae.
  • Scrape or wipe affected surfaces with a soft brush or cloth, then remove the debris so it does not break down and feed the algae again.
  • Brown algae usually clears within two to three weeks once water conditions improve, without the need for chemicals or special treatments.

Test your water and reduce nitrate levels

Brown algae feeds on nitrate, a byproduct of fish waste and decaying food. If your tank has brown algae, nitrate is likely above 20 parts per million (ppm). You can test this with an aquarium test kit — the kind that uses liquid reagents or test strips — available at any aquarium store for $15 to $30. Most kits test ammonia, nitrite, nitrate, and pH in about five minutes.

If nitrate is above 20 ppm, the fastest fix is a 25 to 30 percent water change. Do this weekly until nitrate drops below 20 ppm. At the same time, remove uneaten food after five minutes and vacuum the substrate with a gravel siphon once a week to pull out decaying plant matter and waste that converts to nitrate. If you have live plants, they will also consume nitrate and help keep levels down — adding a few fast-growing plants like water sprite or ludwigia can speed recovery.

Increase water flow and reduce lighting hours

Brown algae grows in still water and low-light conditions where it can settle without being disturbed. Check whether your filter outlet is pointed at the glass or substrate rather than across the tank. Redirect the flow so water moves across all surfaces — this prevents algae from anchoring and makes it easier to remove. If your filter is weak, add an air stone connected to an air pump, which creates gentle circulation and oxygenates the water at the same time.

Lighting duration also matters. Many aquariums are lit for 10 to 12 hours per day, which is longer than most fish need and longer than brown algae prefers. Reduce lighting to 6 to 8 hours per day. Brown algae will slow its growth within days, and you may see it begin to fade within a week. If you have live plants that need more light, use a timer to split the difference — 8 hours of full light plus 4 hours of lower-intensity light — rather than running the tank bright all day.

Physically remove brown algae from surfaces

While you are adjusting water conditions, remove the algae you can see. Use a soft brush, old toothbrush, or algae scraper to gently brush affected glass, rocks, and decorations. For the substrate, a gravel siphon removes both algae and the waste underneath it. Do not use a hard scraper on acrylic tanks, as it will scratch — stick to soft brushes or a plastic credit card edge instead.

After you scrape or brush, remove the loose algae from the tank with a net or by siphoning it out. If you leave it in the water, it will break down and add more nutrients that feed the remaining algae. This step takes 10 to 15 minutes but makes a visible difference and prevents the algae from resettling on clean surfaces.

When brown algae persists despite water changes

If brown algae is still visible after three weeks of weekly water changes, reduced lighting, and improved flow, check whether silicate is the problem. Silicate comes from tap water in some regions and from sand or gravel that has not been rinsed. If your tap water is high in silicate, use reverse osmosis water for water changes, or switch to a substrate that has been pre-rinsed. Silicate-specific test kits are available but less common than nitrate kits — if you suspect silicate, ask your aquarium store to test a sample of your tap water.

In rare cases, brown algae can indicate a new tank that is still cycling. If your tank is less than four weeks old, brown algae is normal and will fade as the tank matures and beneficial bacteria establish themselves. Continue water changes and maintenance, and the algae will clear on its own.

Algae-eating fish and shrimp as a secondary step

Some fish and invertebrates eat brown algae, but they work best after you have fixed the underlying conditions. Otocinclus catfish, plecos, and Amano shrimp all graze on diatoms, but they cannot outpace algae growth if nitrate is still high and lighting is still too long. Add them only after you have reduced nitrate below 20 ppm and cut lighting to 8 hours or less. Once conditions improve, they will keep new algae from building up again.

Do not rely on algae-eaters alone. A pleco or shrimp in a tank with poor water conditions will still struggle, and you may end up with both algae and an overgrown fish that needs a larger tank. Use them as maintenance, not as a cure.

Frequently Asked Questions

Is brown algae harmful to fish?

No. Brown algae does not produce toxins and does not affect water chemistry. Fish can eat it without harm. The only real problem is that it looks bad and signals that nitrate levels are too high, which can stress fish over time if left unchecked.

Can I use chemicals to kill brown algae?

Algaecides are available but unnecessary for brown algae. They kill the algae but do not fix the high nitrate or poor flow that caused it, so the algae returns within weeks. Water changes and maintenance work faster and cost less.

Why did brown algae appear right after I set up my tank?

New tanks are cycling — beneficial bacteria are still establishing themselves, so ammonia and nitrite spike before dropping. During this period, nutrients build up and brown algae thrives. Continue water changes and maintenance, and the algae will fade as the cycle completes, usually within three to four weeks.

Does brown algae mean my filter is not working?

Not necessarily. Brown algae means water flow is weak in certain spots or nutrients are building up faster than the filter can process them. Check that your filter outlet is pointed to create movement across the whole tank, not just in one corner. If flow looks good, the issue is usually excess food or waste, not filter failure.

Can I prevent brown algae in a new tank?

You can reduce it by starting with a pre-rinsed substrate, doing 25 percent water changes twice a week during the first month, and keeping lighting to 6 to 8 hours per day until the tank is established. Brown algae is still common in new tanks, but these steps will make it lighter and shorter-lived.