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

Brown algae (also called diatoms) looks like a thin, dusty brown coating on glass, gravel, plants, and decorations. It is not toxic to fish, but it clouds the view into your tank and can smother slow-growing plants. The good news: brown algae almost always means your water parameters are actually fine—it means you have excess silicates or nitrates feeding it, combined with low water movement in certain spots.

The fastest way to remove it is a combination of three steps: scrub the visible algae off surfaces, increase water flow with your filter or an air stone, and do a partial water change to lower nutrient levels. Most tanks see brown algae retreat within one to two weeks once you address the root cause.

Key Takeaways

  • Brown algae grows when silicates or excess nutrients sit in low-flow areas, so the first step is always to scrub visible algae and improve water circulation.
  • A 25 to 50 percent water change removes dissolved nutrients that feed brown algae and should be done once a week until the algae stops returning.
  • Increasing filter flow or adding an air stone creates movement in dead spots where brown algae thrives, which is often more effective than chemicals.
  • Brown algae does not harm fish or most plants, so you can take time to fix the underlying cause rather than rushing to treat it.

Scrub surfaces and remove visible brown algae

Start by removing what you can see. Use a soft algae brush or old toothbrush to scrub the inside of the glass, rocks, and any hard decorations. For plants, gently rub the leaves between your fingers underwater—brown algae comes off easily because it does not grip surfaces the way green algae does. Do not worry about getting every speck; you are removing the bulk so you can see what is happening in the tank.

If you have a gravel vacuum, use it during your next water change to siphon out debris from the substrate. Brown algae often settles on the bottom, and removing it there prevents it from re-releasing nutrients back into the water. You do not need to tear down the tank—just work through the areas where you see the heaviest coating.

Increase water flow to eliminate dead spots

Brown algae thrives in corners and behind decorations where water barely moves. Check your filter outlet: is it pointing at a blank wall, or is it creating a current across the tank? Redirect the outlet so water flows across the entire tank floor and back toward the intake. This single change stops brown algae from settling in those dead zones.

If your filter flow is already good, add an air stone or small air pump to a back corner. Even gentle bubbling creates enough movement to prevent brown algae from accumulating. Many aquarists run an air stone at night to increase oxygen and water circulation without adding heat from a second filter. The cost is minimal—an air pump and stone run about $15 to $30 total—and the improvement is often visible within days.

Do a partial water change to reduce nutrients

Brown algae feeds on silicates (which come from tap water and some substrates) and excess nitrates (which come from fish waste and uneaten food). A 25 to 50 percent water change removes a large portion of these nutrients in one step. Do this once a week for three to four weeks, or until you stop seeing new brown algae growth.

When you refill, use water at the same temperature as the tank to avoid stressing fish. If you have a gravel vacuum, use it during the change to pull debris from the substrate—this removes both visible algae and the decaying matter that feeds it. The combination of removing nutrients and improving flow is what stops brown algae from coming back.

Check your feeding and maintenance routine

Brown algae often signals that uneaten food is sitting in the tank. Feed only what your fish will finish in two to three minutes, once or twice a day. Leftover food breaks down into ammonia and then nitrate, which feeds brown algae. If you are overfeeding, cutting back is often enough to stop the problem on its own.

Similarly, if you have not cleaned your filter in several weeks, a clogged filter reduces water flow and allows waste to build up. Clean the filter media in old tank water (not tap water, which kills beneficial bacteria) every two to four weeks. A cleaner filter moves water faster and removes more waste before it becomes algae food.

When brown algae returns, look for tap water silicates

If brown algae keeps coming back even after you improve flow and reduce nutrients, your tap water may be high in silicates. This is common in areas with sandy or volcanic soil. You have two options: use a reverse osmosis (RO) filter on your tap water before adding it to the tank, or accept that brown algae will be a minor ongoing issue and straightforward scrub it off monthly.

RO filters are expensive ($150 to $400 for a home unit) and slow, so most hobbyists only use them if brown algae is severe. For most tanks, regular scrubbing and weekly water changes keep it under control without the extra cost. Some aquarists also switch to a substrate with lower silicate content, though this requires a partial tear-down.

Avoid over-treating with chemicals or UV

Algaecides and UV sterilizers can kill brown algae, but they do not fix the root cause—excess nutrients and poor flow. Once the chemical wears off or the UV is turned off, brown algae returns within days. You end up spending money repeatedly on treatments instead of solving the problem once.

The exception is if you have already improved flow and done water changes for two weeks with no improvement. In that case, a single dose of an algaecide designed for diatoms (brown algae) can speed up the process. Follow the label instructions exactly and do a 25 percent water change the day after treatment. But this is a last resort, not a first step.

Frequently Asked Questions

Is brown algae harmful to fish or plants?

Brown algae is not toxic to fish. It can smother slow-growing plants if it gets thick, but most aquatic plants tolerate it. The main problem is that it makes the tank look cloudy and dirty. You can safely leave it alone while you work on the underlying cause.

Why does brown algae appear in new tanks?

New tanks often have brown algae because the nitrogen cycle is still establishing, and water flow patterns are not yet optimized. It usually clears on its own within four to six weeks as beneficial bacteria build up and you dial in your maintenance routine. Speed it up by doing weekly 25 percent water changes and checking your filter flow.

Can I use a pleco or other algae-eating fish to remove brown algae?

Some plecos and other bottom feeders will eat brown algae, but they are not reliable and do not address the root cause. Once the algae is gone, the fish still needs food and space. It is better to fix the flow and nutrients first, then add an algae-eater only if you want one for other reasons.

How long does it take to get rid of brown algae?

Most tanks see a noticeable improvement within one to two weeks of scrubbing, improving flow, and doing weekly water changes. Complete removal usually takes three to four weeks. If brown algae is still heavy after a month, check your tap water for silicates or look for hidden sources of excess nutrients like decaying plants or trapped food.

Should I do a full water change or a partial one?

Do a partial water change—25 to 50 percent weekly. A full water change removes too much of the beneficial bacteria and can stress fish. Partial changes remove nutrients without disrupting the tank's biology, and they are gentler on fish and plants.