Black Beard Algae: What Causes It and How to Actually Kill It
Black beard algae is the one most planted-tank keepers eventually fight. It is tough, it clings to hardscape and slow-growing plant leaves, almost nothing eats it, and it comes straight back if you only remove what you can see.
Despite the name it is a red algae (Rhodophyta), which matters because it explains why the treatments that work on green algae do not touch it.
Identifying it
Dark grey, black, or very deep purple tufts, 3–15 mm, with a distinctly bristly texture. It anchors hardest to the edges of slow-growing plant leaves (anubias and java fern especially), driftwood, filter outflows, and equipment sitting in the current.
The tell that separates it from other algae: drop a tuft in household bleach solution and it turns red or pink. That colour change is diagnostic.
What actually causes it
The single most consistent driver is unstable CO2 and fluctuating water flow, not “too many nutrients,” which is the folk explanation.
- Fluctuating CO2 in an injected tank. Inconsistent levels stress plants, and stressed plants stop competing. This is the number one cause in planted tanks.
- Low or unstable CO2 generally — including tanks with no injection where plant mass has outgrown available carbon.
- High flow areas. BBA thrives in current, which is why it colonises filter outflows and the leaves nearest them.
- Excess organics. Decaying leaves, uneaten food, a dirty filter, infrequent water changes.
- Too much light for the plant growth you have. Long photoperiods or high intensity without matching CO2 and nutrients.
Notice the theme: BBA appears when plants are losing. It is a symptom of an imbalance, and it will keep returning until the imbalance is fixed.
Killing what is there
Spot-treat with liquid carbon or hydrogen peroxide
The reliable method. Turn off the filter, draw liquid carbon (glutaraldehyde) or 3% hydrogen peroxide into a syringe, and apply directly onto the affected areas with the tuft exposed to it. Leave the filter off for 10–15 minutes, then restore flow.
The algae turns red or pink over 24–48 hours, then white, then dies off and is often eaten by fish or shrimp. Seeing it turn red means it is working.
Dose conservatively — both are hard on some plants (vallisneria and mosses particularly) and on shrimp and fish at higher concentrations. Treat a portion of the tank at a time rather than everything at once.
Remove badly affected material
Trim heavily infested leaves rather than treating them. On hardscape, take the piece out and scrub it, or dip it in a 1:20 bleach solution for 2–3 minutes, rinse thoroughly, and dechlorinate before returning it.
Siamese algae eaters
The only fish that reliably eats BBA, and only true Crossocheilus oblongus — they are widely mislabelled in stores, and flying foxes and Chinese algae eaters do not eat it. They also lose interest as they mature and get fed other food. Useful as maintenance, not as a cure.
Stopping it coming back
This is the part that matters, and it is where most people fail.
- Stabilise CO2 if you inject — consistent levels, on before lights, off before lights out. A drop checker held at green all photoperiod.
- Reduce photoperiod to 6–8 hours. Long lighting favours algae over plants.
- Increase water changes to reduce organics — weekly 30–50%.
- Clean the filter regularly. A clogged filter means reduced flow and accumulating organics.
- Grow more fast plants. Stem plants and floaters outcompete algae for nutrients far better than slow anubias and ferns. A tank of only slow growers is a tank that will get BBA.
- Do not reduce nutrients to starve it. This is the classic mistake — it starves your plants first, they stop competing, and the algae wins harder.
A tank with healthy fast-growing plants, stable CO2, moderate light, and regular maintenance does not get black beard algae. Everything else is symptom management.
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