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

Brown algae (diatoms) grows as a thin, slimy brown coating on glass, decorations, and substrate. It is not toxic to fish, but it clouds the tank and signals that something in your water chemistry or tank maintenance needs adjustment. The algae feeds on silicates and excess nutrients — usually nitrate and phosphate — that build up when you overfeed, skip water changes, or have weak water circulation.

Brown algae is most common in new tanks during the first four to six weeks, when the nitrogen cycle is still establishing and nutrients accumulate faster than plants or bacteria can consume them. It also appears in established tanks after you add new substrate, skip maintenance, or reduce lighting. Unlike green hair algae or black beard algae, brown algae usually disappears on its own once you address the root cause — you do not need chemicals or special treatments.

Key Takeaways

  • Brown algae thrives in tanks with high silicate and nutrient levels, so the fastest fix is a 25 to 50 percent water change followed by weekly changes for two to three weeks.
  • Increase water flow with an air stone or powerhead to break up dead spots where nutrients settle and algae spores accumulate.
  • Reduce feeding to once or twice daily in small amounts, because uneaten food decays and raises nitrate levels that feed the algae.
  • Scrub affected surfaces with a soft brush or algae scraper during water changes, but do not use bleach or harsh chemicals that harm beneficial bacteria.
  • If the tank is new, brown algae will often clear on its own within a month as the nitrogen cycle matures and plants establish.

Do a large water change and establish a weekly schedule

Start by removing 25 to 50 percent of the tank water and replacing it with fresh dechlorinated water. This dilutes the silicates and excess nutrients that fuel brown algae growth. If the algae is heavy, repeat the change three days later. Then switch to a weekly 25 percent water change for the next two to three weeks — this keeps nutrient levels low while the tank rebalances.

The timing matters because brown algae spores are always present in the water; they only bloom when conditions favor them. Frequent partial changes remove both the spores and their food source faster than waiting for the tank to self-correct. After three weeks of weekly changes, you can return to your normal schedule — usually 25 percent every two weeks for established tanks — if the algae has cleared.

Increase water movement to prevent nutrient buildup

Brown algae settles in dead spots where water barely moves. Add an air stone connected to an air pump, or position an existing filter outlet to create gentle current across the substrate and glass. If you have a canister filter, angle the return nozzle to sweep the bottom rather than aim it at the surface. A powerhead (small circulation pump) works well in larger tanks, but keep the flow gentle — you want movement, not a current that stresses fish or uproots plants.

Better circulation does two things: it prevents detritus from piling up in corners, and it keeps water chemistry more uniform throughout the tank. Detritus (dead plant matter, uneaten food, fish waste) breaks down into ammonia and then nitrate, which feeds algae. Moving water also helps the filter reach all areas of the tank, so the biological media can process waste more efficiently.

Cut feeding and remove uneaten food when ready

Overfeeding is one of the most common causes of brown algae in established tanks. Feed fish only what they can eat in two to three minutes, once or twice daily. Uneaten food sinks to the substrate, decays, and releases ammonia and phosphate — both of which algae consumes. If you see food on the bottom after five minutes, you are feeding too much.

Use a small net or turkey baster to remove any visible uneaten food during feeding time. This is especially important if you have bottom feeders like plecos or corydoras catfish, because their presence can make you think uneaten food is being consumed when it is actually accumulating. Reducing feeding is often enough to stop brown algae in tanks that are otherwise well-maintained.

Scrub surfaces with a soft brush or algae scraper

Once you have addressed the water chemistry and flow, physically remove the brown algae coating. Use a soft-bristled brush, a dedicated algae scraper, or a clean soft cloth to wipe the glass, decorations, and any hard surfaces. Do not use a scouring pad or steel wool — these scratch acrylic tanks and can damage the protective coating on glass. For substrate, a gentle pass with a gravel vacuum during your water change will remove algae and detritus at the same time.

Do not use bleach, vinegar, or hydrogen peroxide inside the tank, even in small amounts. These chemicals kill beneficial bacteria in the filter and substrate that process fish waste. If you need to clean decorations thoroughly, remove them, soak them in a separate container with a 10 percent bleach solution (one part bleach to nine parts water) for 15 minutes, rinse them very well under running water, and let them air dry before returning them to the tank.

Check your lighting schedule and intensity

Brown algae is less light-dependent than green algae, but excessive lighting still promotes growth. If your lights are on for more than 8 to 10 hours per day, reduce the photoperiod to 6 to 8 hours. This is especially important in new tanks or tanks without live plants — plants consume the nutrients that algae would otherwise use, so without them, algae has less competition.

If you have live plants, keep the lights on for 8 to 10 hours daily and make sure the plants are healthy and growing. Healthy plants outcompete algae for nutrients and will eventually suppress brown algae growth on their own. If plants are wilting or growing slowly, the problem may be insufficient light or missing nutrients (potassium, nitrogen, or trace elements), not too much light.

Use a diatom filter or add silicate-removing resin if the problem persists

If brown algae returns after you have fixed feeding, flow, and water changes, the issue may be high silicate levels in your tap water. Silicates are the primary food source for diatoms. A diatom filter — a small canister that attaches to your existing filter — removes silicates and other fine particles. Run it for a few days to a week until the algae clears, then remove it to avoid removing beneficial particles.

Alternatively, you can add silicate-removing resin to your filter media. This resin is inexpensive and lasts several weeks before needing replacement. If your tap water is naturally high in silicates (common in some regions), using a diatom filter or resin during the first month of a new tank can prevent brown algae from establishing in the first place. After the tank matures and plants establish, silicate removal is usually no longer necessary.

Frequently Asked Questions

Will brown algae harm my fish?

No. Brown algae is not toxic and does not produce harmful gases. Fish can eat it without injury, though most do not prefer it. The main problem is visual — it makes the tank look dirty — and it signals that nutrients are building up faster than the tank can process them.

Can I use algae-eating fish or shrimp to remove brown algae?

Some fish and invertebrates will graze on brown algae, but they are not a complete solution. Amano shrimp and some plecos eat diatoms, but they work best after you have already reduced the nutrient load. If you add them to a tank with active brown algae and do not fix the underlying cause, the algae will return faster than the animals can eat it.

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

In a new tank, brown algae usually clears within two to four weeks once the nitrogen cycle stabilizes and you establish weekly water changes. In an established tank, you should see improvement within one to two weeks of increasing water changes and reducing feeding. If it persists beyond that, check your tap water for high silicates or consider a diatom filter.

Is brown algae a sign that my tank is not cycled?

Not necessarily. Brown algae is common in new tanks during the cycling process because ammonia and nitrite are high, but it also appears in fully cycled tanks with poor maintenance or high nutrient levels. If your ammonia and nitrite are zero and only nitrate is elevated, the tank is cycled — the brown algae is just a sign of excess nutrients, not incomplete cycling.