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

Brown algae (also called diatoms) looks like a thin, dusty brown coating on glass, gravel, plants, and decorations. It is not harmful to fish, but it clouds the tank and signals that something in your water chemistry or tank maintenance needs adjustment. The good news: brown algae is one of the easiest algae problems to solve because the fix addresses the root cause rather than just scraping it away.

Brown algae thrives when silicates are high, water flow is stagnant, or the tank is new. It often appears in the first few weeks after setup or after a long period without water changes. Unlike green algae (which spreads fast) or black beard algae (which is stubborn), brown algae usually disappears within days once you change the conditions that feed it.

Key Takeaways

  • Brown algae grows in tanks with high silicate levels, poor water circulation, or infrequent water changes—fix these and the algae stops growing.
  • Do a 25 to 50 percent water change when ready, then reduce lighting to 6 to 8 hours per day while you address the underlying cause.
  • Increase water flow by repositioning your filter output or adding an air stone, and clean the substrate weekly until the algae clears.
  • If your tap water is high in silicates, use reverse osmosis water or a silicate-removing resin for partial water changes going forward.
  • Brown algae usually disappears within one to two weeks once conditions improve; if it persists, test your water for phosphates and nitrates.

Do an when ready large water change and reduce light

The fastest way to slow brown algae growth is to remove the nutrients feeding it. Perform a 25 to 50 percent water change right away. This dilutes silicates, excess nutrients, and organic waste that diatoms consume. Use a gravel vacuum to stir up the substrate as you drain—brown algae often settles on the bottom, and vacuuming removes both the algae and the debris underneath it.

At the same time, reduce your lighting to 6 to 8 hours per day. Brown algae needs light to photosynthesize, and cutting back slows its growth while you work on the other causes. If your tank has a timer, adjust it now. Do not turn off the light completely—plants and fish still need a day-night cycle—but shorter days make a real difference in the first week.

Improve water circulation throughout the tank

Stagnant water is a brown algae magnet. Dead spots where water does not move allow silicates and waste to settle, creating perfect conditions for diatoms. Check your filter output: is the water flowing across the entire tank, or does it create a current in only one area? Reposition the filter outlet so the flow reaches the back corners and bottom of the tank.

If your filter output is already well-positioned, add an air stone or a second air pump to increase circulation. Even a small air stone in a corner creates movement that prevents algae from settling. You can also use a powerhead (a small submersible pump) if your tank is larger than 20 gallons. The goal is gentle, continuous water movement everywhere—not a strong current that stresses fish.

Clean the substrate and remove visible algae

Brown algae clings to surfaces, so manual removal speeds up the process. Use a soft brush or old toothbrush to gently scrub algae off glass, plants, and decorations. For the substrate, use your gravel vacuum during weekly water changes—this removes both algae and the organic matter it feeds on. Do not scrape aggressively or you risk damaging plants or the tank seal.

If you have live plants, brush them gently by hand or with a soft brush. Brown algae does not harm plants, but removing it improves water flow around the leaves. For plastic plants or decorations, you can soak them in a separate container with a weak bleach solution (one part bleach to ten parts water) for 15 minutes, then rinse thoroughly before returning them to the tank. Never use bleach directly in the aquarium.

Test your tap water and consider a silicate filter

If brown algae returns after two weeks of good maintenance, your tap water may be the culprit. Silicates come from tap water in many regions—they are harmless to fish but feed diatoms. You can have your water tested at a local aquarium store, or buy an inexpensive silicate test kit online. If silicates are above 1 ppm (parts per million), they are likely feeding the algae.

If silicates are high, use reverse osmosis (RO) water for partial water changes going forward. RO water is demineralized and removes silicates, but it also removes beneficial minerals, so mix it 50/50 with tap water for planted tanks or tanks with fish that need minerals. Alternatively, buy a silicate-removing resin cartridge that attaches to your faucet—these are inexpensive and last several months. Some aquarists use both methods for tanks in high-silicate areas.

Check nitrate and phosphate levels if algae persists

Brown algae usually clears within one to two weeks once water flow improves and light is reduced. If it is still present after two weeks, test your water for nitrate and phosphate using a liquid test kit (not strips—they are less accurate). High levels of either nutrient can sustain algae even after you have addressed silicates and flow.

Nitrate builds up from fish waste and uneaten food. If your nitrate is above 20 ppm, increase water change frequency to once or twice per week, and reduce feeding slightly. Phosphate comes from tap water, fish waste, and decaying plant matter. If phosphate is above 1 ppm, remove dead plant leaves when ready, do not overfeed, and consider adding more live plants—they consume phosphate as they grow. Once nitrate and phosphate are under control, brown algae will not return.

Prevent brown algae from coming back

Once your tank is clear, stick to a routine that prevents algae from returning. Do a 25 percent water change every week (or 50 percent every two weeks for smaller tanks). Keep the light on for 8 hours per day, and make sure your filter creates gentle flow throughout the tank. Remove uneaten food within a few minutes of feeding, and take out dead plant leaves as soon as you see them.

In new tanks, brown algae is almost inevitable in the first month—this is normal and not a sign of a problem. The algae usually disappears on its own as the tank matures and beneficial bacteria establish themselves. If you are setting up a new tank, do not panic if you see brown algae; just follow the steps above and it will clear.

Frequently Asked Questions

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

Some fish and shrimp eat brown algae, but they work best as part of a solution, not as the only fix. Amano shrimp and otocinclus catfish will graze on diatoms, but they cannot keep up if the underlying cause (poor flow, high silicates, excess light) is still present. Add them after you have improved water conditions, and they will help maintain the tank.

Is brown algae dangerous to my fish?

No. Brown algae does not produce toxins and does not harm fish or plants. It is purely a cosmetic problem and a sign that water conditions need adjustment. Fish can eat it without ill effects, though most prefer other foods.

Why does brown algae come back after I clean it?

Brown algae returns because the conditions that feed it are still present—usually high silicates, poor water flow, or too much light. Cleaning removes the visible algae but does not fix the cause. Focus on water changes, circulation, and lighting rather than just scrubbing.

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

Most brown algae clears within one to two weeks once you improve water flow, reduce lighting, and do a large water change. If it persists beyond two weeks, test your water for silicates, nitrate, and phosphate to find the remaining cause.

Can I use chemicals to kill brown algae?

Algae-killing chemicals are not necessary and can harm fish and plants. Brown algae responds so well to straightforward maintenance changes (water changes, better flow, less light) that chemicals are overkill. Stick with the methods above and you will see results without risk.