Brown algae is usually a diatom bloom, and it clears faster with water changes and reduced light than with chemicals

Brown algae looks like a thin, dusty coating on glass, gravel, and decorations. It is not the same as green algae. Brown algae (usually diatoms) thrives in new tanks, in tanks with excess silica or nutrients, and in tanks that sit in direct sunlight. The good news: it almost always disappears on its own within two to four weeks once you change the conditions that feed it. You do not need special treatments or expensive products.

Brown algae does not harm fish or plants, but it makes the tank look dirty and can block light from reaching live plants. The fastest way to remove it is a combination of three steps: reduce light, increase water changes, and manually remove what you can see. If you follow these steps consistently, the bloom will fade without chemicals.

Key Takeaways

  • Brown algae is a diatom bloom that appears as a thin brown dust on glass and surfaces, and it clears within two to four weeks with proper tank conditions.
  • Reduce the tank light to six to eight hours per day and move the tank away from windows or direct sunlight to starve the bloom.
  • Perform a 25 to 50 percent water change twice per week to remove excess nutrients and silica that feed the algae.
  • Manually wipe brown algae off glass with a soft cloth or algae scraper, and vacuum the gravel to remove settled particles.
  • Add a small cleanup crew—such as Amano shrimp or otocinclus catfish—to eat algae as it grows, but only after the bloom is under control.

Reduce light to starve the brown algae bloom

Brown algae grows fastest under bright light for long hours. Cut your aquarium light down to six to eight hours per day, and keep the tank away from windows or direct sunlight. If your tank sits near a window, move it or cover the window side with a dark cloth during the day. This single change stops the bloom from spreading and gives you time to address the root cause.

If you have live plants that need more light, keep the light on for the plants but reduce it to the minimum they require. Most aquarium plants do well on eight to ten hours of light per day. Brown algae will fade even if your plants stay healthy on less light than they were getting before.

Do water changes twice per week to remove excess nutrients

Brown algae feeds on excess silica, phosphate, and nitrate in the water. A 25 to 50 percent water change twice per week removes these nutrients faster than the algae can use them. Do not skip this step—it is the most important one.

When you change the water, use a gravel vacuum to siphon debris from the bottom of the tank. Brown algae settles on the gravel, and vacuuming removes it before it can spread. Vacuum slowly over the gravel for 30 to 60 seconds per square foot of tank bottom. If you have a small tank (under 20 gallons), one thorough vacuum per water change is enough. For larger tanks, divide the gravel into sections and vacuum a different section each time.

Use dechlorinated water at the same temperature as your tank. If your tap water is high in silica (common in some regions), consider using bottled water or a reverse-osmosis filter for water changes, but this is only necessary if the brown algae returns after four weeks of regular changes.

Manually remove brown algae from glass and surfaces

While you wait for the bloom to fade, remove what you can see. Use a soft cloth or a plastic algae scraper to wipe the inside of the glass. Do not use a metal scraper on acrylic tanks—it will scratch them. Wipe in one direction to avoid pushing algae around, and rinse the cloth in a bucket of tank water between passes.

For decorations and plants, use a soft toothbrush or a small cloth to gently brush away the algae. Do not scrub hard or you may damage plant leaves. If a decoration is heavily covered, remove it from the tank, rinse it under running water, and scrub it outside the tank. Return it after a few minutes.

You do not need to remove every speck—the goal is to reduce the visible algae while the conditions change. As light decreases and nutrients drop, the algae will stop growing and begin to disappear on its own.

Check your filter and substrate for excess silica

Brown algae blooms are common in brand-new tanks because gravel and filter media release silica as they break down. If your tank is less than four weeks old, this is normal and will pass. If your tank is older and the brown algae returns after you clear it, your gravel or filter media may be the source.

New gravel should be rinsed thoroughly before it goes in the tank. Rinse it in a bucket under running water, stirring it with your hand, until the water runs clear. This removes dust and loose particles. If you did not rinse your gravel when you set up the tank, a large water change now will help, but you may need to replace some of the gravel later if the algae keeps coming back.

Some filter media (especially cheap activated carbon) releases silica into the water. If you have been using the same filter media for more than a month and the brown algae persists, replace the media with a new batch and see if the problem improves.

Add cleanup crew fish or shrimp after the bloom clears

Once the brown algae is mostly gone (after two to four weeks), you can add fish or shrimp that eat algae and help prevent future blooms. Amano shrimp are the most effective—they eat brown algae constantly and do not uproot plants. Otocinclus catfish also eat brown algae and stay small. Both are peaceful and work in most community tanks.

Do not add a cleanup crew while the bloom is still heavy. They will not eat fast enough to keep up, and they may starve. Wait until the algae is light enough that you can see the glass clearly again, then add one or two shrimp or fish. They will eat any algae that tries to return and keep your tank cleaner going forward.

Avoid plecos and Chinese algae eaters if your tank is small. Plecos grow very large and produce a lot of waste, which feeds algae. Chinese algae eaters become aggressive as they age and may damage other fish.

When to use algae treatments or UV filters

You do not need chemical algae treatments for brown algae. They are expensive, can harm some plants and fish, and do not address the root cause—the algae will return if you do not change the light and nutrients. Stick with the steps above for two to four weeks before considering any product.

If brown algae returns repeatedly after you have reduced light and done water changes, a UV sterilizer may help. A UV sterilizer kills algae spores as water passes through it, preventing new blooms. They cost between $30 and $100 depending on tank size, and they use electricity, but they work well for stubborn cases. You still need to do water changes and reduce light—a UV sterilizer is a tool to use alongside those steps, not instead of them.

Frequently Asked Questions

Is brown algae harmful to fish or plants?

Brown algae does not poison fish or plants. It can block light from reaching live plants if it gets very thick, which may slow plant growth. For fish, it is purely a cosmetic problem. You can leave it alone if it does not bother you, and it will fade on its own.

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

With reduced light and twice-weekly water changes, brown algae usually clears within two to four weeks. If you do only one water change per week or leave the light on for 12 hours per day, it may take longer. The faster you change the conditions, the faster it disappears.

Can I use bleach or hydrogen peroxide to clean brown algae?

Do not use bleach in your tank—it will kill fish and plants. Hydrogen peroxide at very low doses (a few drops per gallon) is sometimes used to treat algae, but it is risky and unnecessary. Water changes and reduced light work just as well and are much safer. Stick with those methods first.

Why does brown algae come back after I clean it?

Brown algae returns if the conditions that feed it are still present: too much light, excess nutrients, or high silica. If it comes back after you have reduced light and done water changes for four weeks, check your gravel and filter media. You may need to replace them or do larger water changes if your tap water is high in silica.

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

Live plants actually help prevent brown algae by competing for nutrients. Keep your light on for eight to ten hours per day (the minimum most plants need), do water changes twice per week, and add a small amount of fertilizer if your plants start to yellow. The plants will grow faster and use up the nutrients before algae can, and the brown algae will clear faster.