We are reader supported. When you purchase through links on our site, we may earn an affiliate commission. Also, as an Amazon affiliate, we earn from qualifying purchases.
Black algae can make a beautiful aquascape look tired and messy very quickly. If you see dark tufts clinging to your filter outlet, plant leaves, or wood, you are likely dealing with black beard algae (often called BBA). The good news is that you can beat it with a clear plan. This beginner-friendly guide explains what black algae really is, why it appears, and how to remove it for good using safe, practical steps. You will also learn how to keep it from coming back through better lighting, nutrients, CO2, and maintenance routines.
What “black algae” actually is
Meet black beard or black brush algae (BBA)
When aquarists say “black algae,” they usually mean black beard algae, also known as black brush algae. It belongs to the red algae group, even though it looks dark gray to black in most tanks. BBA forms short, fuzzy tufts that sway in the current. You will often see it on hardscape, filter outlets, slow-growing plant leaves (like Anubias), and along glass edges with steady flow.
Fresh growth can look deep green or gray, then turn black as it matures. When BBA is dying after treatment, it often shifts to reddish, purple, or pink before disappearing. That color change is a useful sign that your strategy is working.
Common look-alikes and how to tell the difference
Staghorn algae can be confused with BBA. It forms longer, branching strands that look like antlers and appear gray to pale green. It is more stringy and less bushy than BBA.
Diatoms (brown algae) coat surfaces in a dusty brown film, especially in new tanks. They wipe off easily with your finger and do not form tufts.
Cyanobacteria (blue-green algae) forms slimy sheets that can look very dark in low light, but it peels away in sheets and smells musty. That is a key difference from the brush-like texture of BBA.
Green spot algae is hard green dots on glass and leaves that can look almost black in shade. It is very tough to scrape off, unlike the fuzzy feel of BBA.
Is black algae dangerous?
Black algae is not directly toxic to fish, but it can smother plants, block light, and stress the whole system. Once it grabs hold, it spreads quickly because it thrives in the same nutrient-rich water that fish and plants need. The goal is not just to remove it, but to fix the conditions that helped it appear.
Why black algae appears
Unstable or low CO2 is the number one trigger
In planted aquariums, inconsistent CO2 is the most common cause of BBA. If you inject CO2 and your level swings during the day, or you have weak surface agitation that causes fluctuating gas exchange, plants become stressed. Stressed plants leak organics and stop competing well, which opens the door for BBA. Even in non-CO2 tanks, low carbon availability and unstable conditions can fuel BBA outbreaks.
Too much or inconsistent light
Long or intense photoperiods make every kind of algae more confident. BBA often flourishes in tanks with bright lighting, especially when light intensity or timing changes frequently. Strong light with poor plant growth is a classic recipe for algae, because light without balanced CO2 and nutrients becomes “empty energy” that algae can exploit.
Excess organics and weak flow
Overfeeding, dirty filters, decaying leaves, and mulm trapped in low-flow corners all feed BBA. It loves to colonize areas where water movement is steady but not strong enough to keep surfaces clean, like filter lips, spray bars, and slow-growing plant margins. Flow that is too gentle allows detritus to settle, and that detritus becomes algae food.
Nutrient imbalance myths and reality
Many people blame phosphate or nitrate as the single cause. In reality, BBA is more about instability than a specific nutrient number. Low nutrients can stunt plants and still lead to algae. A better goal is stable, non-zero nutrients that match your light and plant mass. Balanced fertilization helps plants outcompete algae consistently.
Quick wins: remove what you see
Manual removal and strategic pruning
Start with what you can physically remove. Trim affected leaves rather than trying to scrape them clean. Plants like Anubias, Bucephalandra, or Java fern can be trimmed back to remove the worst areas. Restart your filter and siphon out loose tufts during water changes so the algae does not settle elsewhere. The cleaner your tank is, the easier it will be for your long-term fixes to work.
Spot treatments that work
Hydrogen peroxide at 3 percent strength can be used to spot treat BBA. Turn off your filter and pumps, use a syringe to gently apply peroxide directly onto the algae, and let it sit for a few minutes before restarting flow. A conservative in-tank amount is up to about 1 milliliter per 10 liters of water per session. Repeat every few days. Sensitive species like shrimp, some snails, mosses, and Vallisneria may be affected by heavy dosing, so go slow and watch your livestock.
Liquid carbon products (glutaraldehyde-based) can also be spot dosed on BBA with pumps off. Follow the label’s daily dose for the whole tank and apply a portion of that dose directly to algae patches. Many aquarists see BBA turn reddish and then white within a week. Be careful with mosses, liverworts, and Vallisneria, which can melt from strong exposure. Never exceed the manufacturer’s guidelines.
Deep cleaning hardscape and equipment
Rocks, filter pipes, and plastic parts can be soaked outside the tank. A dilute bleach bath, such as one part unscented bleach to nineteen parts water, can clear BBA from hard items. Soak for several minutes, then rinse extremely well and soak in water with a heavy dose of dechlorinator. For driftwood, avoid long bleach soaks; pouring boiling water over the surface or spot-scrubbing with peroxide is safer because wood absorbs chemicals.
What about algaecides and UV?
Algaecide products can knock down visible algae but do not fix the root cause. Used repeatedly, they may stress fish or invertebrates. A UV sterilizer can reduce free-floating algae and some spores, improving water clarity. It will not cure BBA on surfaces by itself, but it can be a helpful add-on once you have corrected light, nutrients, and CO2. Focus on stability first; tools like UV are optional extras.
Long-term fix: stabilize your system
Dial in your light
Set a consistent photoperiod of 6 to 8 hours for most tanks. If you previously ran 10 to 12 hours, reduce gradually by an hour every few days. If your light has dimming, consider starting at a moderate intensity rather than full power, especially on shallow tanks. Avoid sudden changes to light timing or strength, as plants need time to adjust. A simple, predictable schedule beats complex ramps and irregular on-off cycles for algae control.
Get CO2 right, with or without injection
For CO2-injected tanks, aim for a stable CO2 level from just before lights-on to just before lights-off. Many aquarists target a pH drop of about one unit from degassed water to lights-on CO2 levels, which often corresponds to roughly 20 to 30 ppm of CO2. A drop checker showing light green during the photoperiod is a practical cue. Ensure good surface agitation to maintain oxygen while keeping the CO2 level steady; a gentle ripple is ideal.
For non-CO2 tanks, choose lower light and focus on consistent fertilization and water changes. Adding floating plants can provide shade and natural nutrient uptake. Avoid big swings in water chemistry, stocking, or feeding. Stability is your ally when you do not supplement carbon.
Feed plants, not algae
Provide regular, balanced nutrients that match your light and plant mass. A simple target for many planted tanks is nitrate between 10 and 20 ppm, phosphate between 0.5 and 2 ppm, and adequate potassium and trace elements from a trusted fertilizer. The exact numbers are less important than keeping them stable and non-zero. Underfeeding plants makes them weak and prone to algae; overfeeding fish increases organics. Find the middle and hold it steady.
Maintain with purpose
Weekly water changes of 30 to 50 percent help remove dissolved organics and reset nutrients. Clean your filter media gently in removed tank water so you do not crash beneficial bacteria. Vacuum detritus from the substrate, especially in low-flow pockets and under hardscape. Trim and replant overgrown stems to keep fresh growth strong and exposed to light.
Improve flow and filtration
Good circulation keeps surfaces clean and delivers CO2 and nutrients to leaves. Aim to eliminate dead spots where debris accumulates. Adjust the direction of your filter outlet or add a small powerhead to move water through dense plant sections. A general guide is total turnover of five to ten times the tank volume per hour, but the real measure is even, gentle movement everywhere, not just a strong jet in one corner.
Helpful livestock and plants
True Siamese algae eaters
True Siamese algae eaters (Crossocheilus siamensis) will graze on young, soft BBA and are one of the few fish that sometimes eat it. They prefer slightly cooler to midrange tropical temperatures and are active schooling fish. Avoid look-alikes like the Chinese algae eater or flying fox, which may become aggressive or ignore algae as they mature. Fish are helpers, not a cure. If conditions stay unstable, even the best algae eaters will not keep up.
Shrimp and snails
Amano shrimp and some snails, like nerites, will happily clean soft algae and biofilm but usually ignore mature BBA. However, once you weaken BBA with peroxide or liquid carbon, these cleanup crews often finish the job by eating dying tufts. If you plan to use chemical spot treatments, dose carefully to protect invertebrates, and consider treating small areas at a time.
Fast growers to outcompete algae
Adding fast-growing plants helps soak up nutrients before algae can claim them. Stems like water sprite, hornwort, or Hygrophila grow rapidly under moderate light. Floating plants such as Salvinia, frogbit, or water lettuce shade the tank and absorb excess nutrients through their roots. Strong plant growth is the most natural long-term defense against BBA.
A week-by-week action plan
Week 1: Identify and reset
Confirm that you are dealing with BBA by checking for brushy tufts that sway in the current. Reduce the photoperiod to a stable 6 to 8 hours. Clean the tank thoroughly, prune affected leaves, and siphon out debris. If you inject CO2, verify that your CO2 level is stable during the entire photoperiod. Start gentle spot treatments on the worst patches and monitor livestock closely.
Week 2 and 3: Stabilize and support plant growth
Continue pruning and spot treatments on new or remaining tufts. Adjust your fertilizer routine to maintain stable, non-zero nutrients. Add or reposition flow if you see detritus collecting. Consider adding fast-growing plants or floating plants to increase competition for nutrients. Watch for signs of dying BBA turning reddish or pink, which shows progress.
Week 4 and beyond: Prevent the comeback
As the tank stabilizes, BBA should stop spreading and begin to recede. At this point, reduce spot treatments and focus on routine: consistent lighting schedule, weekly water changes, and gentle filter maintenance. Keep trimming plants to encourage fresh growth. If BBA reappears, check your CO2 and lighting first, and look for new dead spots or debris traps that need attention.
Prevention and quarantine
Dip protocols for new plants and hardscape
BBA often hitchhikes on new plants or decor. For hardy plants like Anubias or stems, a quick dip in a mild bleach solution can reduce risk. Use one part unscented bleach to nineteen parts water for 60 to 120 seconds, then rinse well and soak in water with dechlorinator. Do not bleach delicate plants, mosses, or ferns; instead, rinse them thoroughly and quarantine. For rocks and plastic equipment, longer soaks are possible, followed by heavy dechlorination and rinsing. Avoid long bleach exposure on wood. Peroxide rinses and boiling water are safer for wood.
Quarantine and observation
If you can, keep new plants in a separate container or small tank for a couple of weeks. Observe for algae and snails. Provide light and a small amount of nutrients so plants stay healthy during quarantine. Healthy plants resist algae better when finally added to your display tank.
Feeding, stocking, and bio-load
Feed fish only what they will eat in a couple of minutes. Remove uneaten food. Avoid overcrowding, which increases waste and stresses fish. A balanced bio-load makes filtration, water changes, and algae control far easier. Remember that stability matters more than any single parameter number; small, regular habits beat big, irregular interventions.
Troubleshooting and FAQs
BBA keeps coming back. What am I missing?
Persistent BBA almost always points to instability. Check that your light is not too strong for your plant mass. Confirm your photoperiod is consistent. If you run CO2, make sure the level is steady during the entire photoperiod and reaches your target before lights-on. Improve flow where tufts keep appearing and remove detritus. Keep nutrients stable and non-zero.
Will black algae harm my fish?
It will not directly poison fish, but it can trap detritus, lower plant health, and reduce oxygen at night if plant mass is large and struggling. Poor conditions tied to BBA outbreaks, such as high organics and low flow, are what stress fish. Fixing those factors benefits both fish and plants.
Can I just turn the lights off for three days?
A blackout hurts many algae types, but BBA often survives and returns if conditions are unchanged. Blackouts can help cyanobacteria more than BBA. Use a blackout only as a short-term reset while you fix light, CO2, and nutrients. Without those changes, the algae will rebound.
Do phosphate removers fix BBA?
Chasing a single nutrient rarely solves BBA. Stripping phosphate too low can starve plants and make algae worse long term. Instead, hold nutrients in a healthy range. Stable, well-fed plants outcompete algae better than plants kept on a nutrient crash diet.
Does RO water help?
Reverse osmosis water can reduce unknown minerals and give you control over hardness, but it is not a cure. If you use RO, remineralize properly and keep routines steady. BBA control still depends on stable CO2, sensible light, and clean water.
Real-world examples of balance
High-tech planted tank
In a CO2-injected setup with bright light, aim for a consistent pH drop at lights-on, even flow across the entire scape, and a regular fertilization schedule. Keep the photoperiod around 6 to 7 hours at first. Trim aggressively to keep leaves free from shade and to encourage new growth. In these tanks, even a small CO2 instability can invite BBA, so reliability of your diffuser, regulator, and timer matters a lot.
Low-tech community tank
In a non-CO2 tank, dim the light or raise the fixture to reduce intensity. Shorten the photoperiod and add floating plants to shade the water. Fertilize lightly but consistently. Choose hardy plants like crypts, Java fern, and Anubias, and avoid overstocking. Weekly water changes and careful feeding will keep nutrients stable and organics low. BBA can still appear in low-tech tanks but is easier to manage when light is modest and maintenance is regular.
Safety notes for treatments
Protecting fish and invertebrates
When spot treating with peroxide or liquid carbon, always start small, observe, and increase only if livestock show no stress. Turn off filters to keep the treatment local and restart after a few minutes. Treat a few areas per session rather than the whole tank at once. Provide extra aeration during and after treatment if needed. Remember that the best long-term treatment is a stable environment, not chemicals.
Handling chemicals and equipment
Use gloves and eye protection when working with bleach or concentrated liquid carbon. Label syringes and containers clearly and keep them away from children and pets. Rinse equipment thoroughly and neutralize bleach with plenty of dechlorinator before returning items to the aquarium. Never mix chemicals together. If you are unsure, err on the side of caution and use hot water and elbow grease instead.
Putting it all together
A simple formula you can trust
Black algae fades when plants are strong, water is clean, and the tank runs on a predictable routine. The formula is simple: reduce and stabilize light, keep CO2 steady if you use it, maintain non-zero but balanced nutrients, improve flow, and remove organics. Add gentle spot treatments as needed to help weak patches disappear. Support your plants with regular trimming, and let helpful fish or shrimp assist, but do not rely on them as the only fix.
Signs you are winning
Within one to three weeks, BBA should stop spreading. Treated tufts will change color to red, purple, or pink, then shrink. New plant growth will be clean and vibrant. Water will look clearer. Maintenance will feel easier because you are no longer fighting the same problems every week. When you see these signs, keep your routine steady and resist the urge to make big changes.
Conclusion
From frustration to a stable, beautiful aquarium
Black algae can be stubborn, but it always responds to a balanced approach. Identify it correctly, remove what you can see, and then fix the root causes: unstable CO2, too much or inconsistent light, excess organics, and weak flow. Keep nutrients steady and give plants what they need to thrive. With a clear plan and a few weeks of patience, your aquarium will shift from algae-dominated to plant-driven, and your fish will enjoy a cleaner, healthier home. The best part is that the habits you build while beating BBA are the same habits that prevent future outbreaks. Consistency wins, and your tank will show it.