Brown algae grows when your tank has too much light, excess nutrients, or poor water circulation
Brown algae — also called diatoms — appears as a thin, slimy brown coating on glass, gravel, plants, and decorations. It is not a sign of a dirty tank. Brown algae thrives in new aquariums and established ones when three conditions line up: excess silicates in the water, high nitrate or phosphate levels, and light exposure of 8 to 10 hours or more per day. The algae feeds on these nutrients and uses light to grow.
Brown algae is harmless to fish and plants, but it clouds the view and can smother slow-growing plants if left unchecked. The good news is that it almost always disappears on its own within a few weeks once you address the root cause. You do not need chemicals or special treatments — just adjustment of light, water changes, and sometimes a bit of manual removal.
Key Takeaways
- Brown algae feeds on silicates, nitrates, and phosphates, so it thrives in new tanks and tanks with leftover fish waste or uneaten food.
- Reducing light to 6 to 8 hours per day is the fastest way to slow brown algae growth while you address nutrient levels.
- Weekly 25 to 30 percent water changes remove excess nutrients and silicates that fuel the algae.
- Brown algae is not toxic and will not harm fish or plants, but it can block light from reaching slow-growing species.
- Improving water circulation with a powerhead or adjusting filter flow helps prevent dead spots where algae accumulates.
Why new aquariums get brown algae almost when ready
New tanks are brown algae magnets because the filter has not yet built up enough beneficial bacteria to process fish waste into less harmful compounds. Fish produce ammonia, which bacteria convert to nitrite, then to nitrate. In a new tank, nitrate accumulates faster than plants can consume it. Brown algae sees this excess nitrate as a food source and blooms within days.
Tap water itself often contains silicates — a nutrient that brown algae uses to build its cell walls. In a new tank, silicates have not yet been consumed by plants or removed by water changes, so they sit in the water column waiting to fuel algae growth. This is why brown algae is so common in the first month, even in tanks that are otherwise well-maintained.
The cycle usually breaks on its own. As the filter matures and plants grow, they consume more nitrate and silicates. Brown algae starves and disappears. You can speed this up by doing larger or more frequent water changes during the first month.
How excess light pushes brown algae growth
Aquarium lights that run 10 to 12 hours per day give brown algae everything it needs to thrive. The algae uses light energy to convert nutrients into growth. Reduce light to 6 to 8 hours per day and you cut off one of the three legs holding up the algae.
Many aquarium owners run lights longer than necessary because they assume plants need constant light. Most aquatic plants do well on 6 to 8 hours per day. Fish do not need light at all — they have no eyelids and rest when it is dark. Shortening the photoperiod (the number of hours the light is on) is one of the easiest fixes and often shows results within a week.
If you use a timer, set it to turn on in the morning and off in the evening. If you do not have a timer, turn the light off manually each day. Even reducing from 12 hours to 8 hours makes a visible difference in brown algae growth.
Nutrient buildup from overfeeding and poor maintenance
Fish waste and uneaten food break down into ammonia, which eventually becomes nitrate. Nitrate does not hurt fish in moderate amounts, but it feeds algae. Tanks that are overfed or have infrequent water changes accumulate nitrate faster than plants can use it, and brown algae blooms as a result.
Overfeeding is the most common cause. Feed fish only what they can eat in 2 to 3 minutes, once or twice per day. Uneaten food sinks to the gravel, rots, and releases ammonia. Even small amounts of excess food add up over weeks. If you are unsure whether you are overfeeding, feed less — fish are more forgiving of underfeeding than overfeeding.
Decaying plant matter also releases nutrients. Remove dead leaves and stems as soon as you notice them. Trim plants regularly so they stay healthy and consume nutrients efficiently. A well-planted tank uses up nitrate faster than a sparsely planted one, which is why heavily planted tanks often have less algae.
Poor water circulation and dead spots
Brown algae accumulates in corners and behind decorations where water movement is slow. Stagnant water allows nutrients to pool and algae to settle without being disturbed. Improving circulation breaks up these dead spots and keeps nutrients distributed throughout the tank where plants can use them.
Check your filter output. If water is flowing in only one direction, add a small powerhead or adjust the filter outlet to point across the tank rather than straight up. Even gentle circulation prevents algae from settling on the substrate and glass. You do not need strong current — just enough movement so that no area of the tank sits completely still.
Rearranging decorations occasionally also helps. Moving rocks or plants breaks up algae buildup and exposes it to the water column, where it is easier to remove manually or for fish to graze on.
How to remove brown algae and prevent it from returning
Brown algae wipes off easily with a soft cloth or algae scraper. Wipe the glass, rocks, and decorations during your regular maintenance. The algae does not grip tightly like green spot algae, so a gentle swipe removes most of it. You do not need to remove every trace — focus on areas that block your view.
Some fish and invertebrates eat brown algae. Otocinclus catfish, plecos, and Amano shrimp graze on diatoms throughout the day. If you add these to your tank, they will help keep brown algae under control. However, they are not a substitute for addressing the root cause — if nutrients and light are still high, the algae will grow faster than the fish can eat it.
After you remove the algae, stick to the prevention steps: reduce light to 6 to 8 hours, do a 25 to 30 percent water change weekly, feed sparingly, and improve circulation. Brown algae should fade within 2 to 4 weeks. If it returns, check whether you have drifted back to longer light hours or skipped water changes.
When brown algae signals a deeper problem
Brown algae itself is not dangerous, but persistent brown algae can mean your tank has a larger issue. If brown algae returns every few weeks despite your efforts, check your tap water. Some municipal water supplies contain high levels of silicates or phosphates. If that is the case, a reverse osmosis filter or phosphate-removing media can help, but most aquarists straightforward accept occasional brown algae as part of the hobby.
Persistent brown algae can also mean your filter is not keeping up with the bioload. If you have too many fish for your tank size, or if your filter is undersized, nitrate will accumulate no matter how often you change the water. In that case, consider upgrading the filter or reducing the number of fish.
Brown algae that appears alongside green algae or black beard algae suggests multiple nutrient problems. Address all three at once: reduce light, increase water changes, reduce feeding, and improve circulation. Different algae types respond to different fixes, so tackling the fundamentals helps with all of them.
Frequently Asked Questions
Is brown algae harmful to fish?
No. Brown algae does not produce toxins and does not harm fish. It can smother slow-growing plants if it builds up heavily, but fish will not be affected. Some fish and shrimp actually eat it as a food source.
Can I use algae-killing chemicals to get rid of brown algae?
You can, but you do not need to. Brown algae almost always disappears within a few weeks if you reduce light and do regular water changes. Chemicals can harm plants and invertebrates, so they are a last resort. Addressing the cause is faster and safer.
Why does brown algae come back even after I clean it off?
Brown algae returns because the conditions that caused it are still present. If you wipe it away but keep the light on for 12 hours and skip water changes, the algae will grow back within days. Fix the light schedule and nutrient levels, and it will not return.
Do I need to do more water changes to get rid of brown algae?
Increasing water changes from weekly to twice weekly can speed up the process, especially in new tanks. Larger water changes (30 to 50 percent instead of 25 percent) remove more silicates and nitrates. However, consistent weekly changes are usually enough if you also reduce light and improve circulation.
Will plants help reduce brown algae?
Yes. Fast-growing plants like water sprite, ludwigia, and rotala consume nitrate and silicates quickly, which starves brown algae. Adding more plants or switching to faster-growing species can help prevent brown algae from returning. Slow-growing plants like anubias and java fern are less effective at competing with algae.