Why Is Black Beard Algae on My Plants?

Audouinella (black beard algae, also called black brush algae or BBA) belongs not to the green algae but to the Rhodophyta — the red algae. In the aquarium it appears as dense black, dark grey, or purple tufts 2–5 mm long, anchored firmly to plant leaf edges, driftwood, hardscape, and filter inlets. It is a persistent alga precisely because its root cause is almost never what keepers first suspect — excess phosphate or nitrate — but CO2 instability, which is invisible to the eye and undetectable without a drop checker.

Part of the Complete Aquatic Plants Guide.

Main Causes

The dominant cause in CO2-injected tanks is fluctuation rather than absence. As Walstad (2013) discusses in Ecology of the Planted Aquarium, planted-tank algae problems commonly trace to imbalances in the carbon cycle rather than nutrient loading. A tank with stable dissolved CO2 in the 20–30 ppm range rarely develops serious BBA. A tank where CO2 drops overnight because the solenoid is untimed, where the cylinder runs low undetected, or where output varies hour to hour across the photoperiod produces it reliably. The broader framework for identifying algae types by their underlying cause is set out in Algae Diagnosis and Control.

How to Identify the Problem

BBA is sometimes confused with staghorn algae and with dark biofilm on equipment. The table below separates the three most common dark algae types.

Attachment firmness is the most reliable field character. BBA filaments anchor into the substrate or plant tissue and do not yield to gentle pressure. Green hair algae and newly settled diatom films both come away easily. If dark material stays put when you rub it lightly with a finger, BBA is the working diagnosis.

Risk and Severity

BBA grows slowly — an infected anubias leaf takes weeks to become heavily engulfed — and this slow pace is precisely what makes it a chronic problem. Keepers under-respond because nothing looks urgent, and by the time the tank looks bad the algae has been spreading for one to two months.

The greatest risk is to slow-growing epiphytes. Anubias nana and Anubias barteri are the most consistently affected species in practice: their leaves persist for years, providing the extended, undisturbed surface that BBA colonises preferentially. Java fern (Microsorum pteropus) carries the same vulnerability. Once BBA establishes on a mature anubias leaf, the tufts shade the leaf surface and, at high density, the plant eventually abandons the leaf. Old anubias leaves that have hosted BBA for six months or more rarely recover fully after treatment; the practical response is to cut them back to the rhizome and wait for fresh growth.

Fast-growing stem plants — Rotala, Hygrophila, Ludwigia — cycle leaves quickly enough that BBA rarely establishes before the affected leaf is replaced by new growth, provided the underlying CO2 instability is resolved.

Solutions and Actions

Address these in order: physical removal first, chemical spot-treatment of remnants, biological grazers as a management layer, and finally the systemic CO2 and flow correction that prevents regrowth.

Physical removal:

Remove as much BBA as possible manually during a partial water change. Scrub filter inlets, outflow nozzles, and hardscape with a brush. Cut and discard heavily infested anubias and Java fern leaves rather than leaving them in the tank — decomposing material with BBA attached re-seeds the water column with viable algal cells.

Spot-treatment with hydrogen peroxide:

After physical removal, apply 3% hydrogen peroxide at approximately 1 ml per litre of tank volume directly to the remaining BBA tufts via syringe. Lower the water level first to expose the algae, or apply through the water column with the filter running slowly. Leave for 30 minutes, then perform a 30–40% water change. BBA tufts turn red or pale pink within 24–48 hours as the cells die — this colour change reflects the red algal pigments becoming visible as the black cell contents break down. Hydrogen peroxide is the recommended first option for tanks containing shrimp.

Spot-treatment with liquid carbon (glutaraldehyde):

Liquid carbon products containing glutaraldehyde can be applied directly to BBA tufts via syringe with the filter off for 10–15 minutes. Several important cautions apply. Vallisneria species are highly sensitive to glutaraldehyde and will melt or die even at low doses — do not use this method if Vallisneria is present. Dwarf shrimp (Neocaridina and Caridina species) are sensitive in proportion to concentration, and spot-application in an established shrimp tank carries genuine risk. Whole-tank dosing at the liquid carbon supplementation rate has little effect on BBA and is not recommended. Confine any glutaraldehyde use to precise, targeted syringe application.

Biological grazers — Siamese algae eater and amano shrimp:

Crossocheilus oblongus (Siamese algae eater) is among the few fish species documented to consume BBA. Young fish under approximately 8–10 cm graze it actively; older adults often ignore BBA once other food is available. Two or three juveniles in an appropriately sized planted tank (minimum 120 litres) offer useful long-term management rather than rapid clearance. They will not clear an existing heavy infestation on their own and require the underlying CO2 problem to be corrected first.

Caridina multidentata (amano shrimp) graze on young, soft BBA growth and provide a useful preventative layer in a well-maintained tank. Established tufts are generally ignored. Both species are most effective once CO2 has been stabilised and the rate of new BBA production has slowed.

Systemic correction — CO2 and flow:

Physical removal and chemical treatment address existing BBA. Without systemic correction, regrowth typically recurs within 3–8 weeks.

• Stabilise CO2. Use a solenoid timed to the photoperiod so injection runs with lights, not overnight. Monitor with a drop checker: lime green indicates approximately 20–30 ppm dissolved CO2. A deep blue reading two hours after CO2 starts flowing means the bubble rate is too low; a yellow reading at lights-on means CO2 ran overnight. The full setup and calibration process is covered in CO2 Injection Setup.
• Improve circulation. Dead spots behind hardscape, under broad anubias leaves, or near the substrate surface are prime BBA sites. Adjust the filter return or add a small powerhead to reach low-flow areas.
• Reassess light intensity. At high PAR (above approximately 150 µmol/m²/s at the substrate), plants' CO2 demand often exceeds what the system delivers consistently. Reducing the photoperiod from 10 hours to 8 hours lowers total daily carbon demand without reducing light quality. The relationship between PAR, photoperiod, and CO2 demand is discussed in LED Lighting for Planted Tanks.

Prevention

• Use a drop checker. A calibrated drop checker provides continuous, visible feedback on dissolved CO2. Without one, CO2 management is guesswork. Read it at lights-on to establish the overnight baseline, then again 30–60 minutes into the photoperiod to confirm CO2 has reached target.
• Time CO2 injection to the photoperiod. Injection should begin 30–60 minutes before lights-on and stop at lights-off. CO2 running overnight, when plants are not photosynthesising, produces exactly the daily swing — elevated CO2 at dawn crashing as plants ramp up uptake — that correlates most strongly with BBA outbreaks.
• Maintain turnover throughout the tank. Aim for 5–10× tank volume per hour in planted systems. Map low-flow areas by observing how fine particulate matter settles and redirect outflows to cover them.
• Trim ageing leaves proactively. Anubias leaves older than 12–18 months are the most likely colonisation sites. Cutting them back to the rhizome before BBA establishes prevents the slow, demoralising process of treating heavily infested mature growth.
• Do not rush new-tank stocking. CO2 and oxygen dynamics in a new planted tank are unstable for the first 4–8 weeks. Adding livestock before plants are established and CO2 output is stabilised creates ideal BBA conditions.

Common Mistakes

1. Treating with a blackout alone. BBA tolerates darkness better than green water or green dust algae. A 3-day blackout may weaken mild growth but does not clear an established infestation and does nothing to address the underlying cause.

2. Adding more livestock to control the algae. BBA is a CO2 and flow problem. Increasing bioload raises CO2 demand and organic waste, typically making conditions worse. Siamese algae eaters are a useful management tool, not a substitute for fixing the gas exchange problem.

3. Dosing the whole tank with liquid carbon (glutaraldehyde) at the label rate. Full tank dosing is ineffective against BBA and carries real risk to Vallisneria and shrimp. Spot-application via syringe is the only method with an acceptable risk-to-benefit ratio.

4. Reading the drop checker at the wrong time. Checking CO2 mid-photoperiod, after two hours of plant uptake, misses the overnight CO2 peak and the morning drawdown. Read the checker at lights-on to understand the actual fluctuation range the tank is experiencing.

5. Repeating spot-treatment without fixing the cause. Hydrogen peroxide treatment works. Applied repeatedly in isolation, without stabilising CO2 or improving flow, BBA returns within weeks. Treat the current growth, then invest equal effort in diagnosing and correcting the mechanism.