Black beard algae: the red algae behind it and what sources advise
Short answer
Black beard or brush algae are red algae, which Aquasabi attributes to genera such as Audouinella and Rhodochorton; they look dark green to black but turn reddish in alcohol [1, 4]. Aquasabi links them to organic pollution in heavily stocked tanks, to iron imbalance in planted tanks and, as a stated belief, to low CO2 [1]. Control combines fixing those conditions with removal; Seachem reports hobbyist observations of less beard algae when dosing Excel but does not market it as an algicide [6].
What it is
Black beard algae, also called brush algae, form dark tufts that look like a beard or a brush on leaf edges, equipment, wood and even grains of substrate, and they cling stubbornly [1]. Despite the name and the color, which ranges from dark green through gray to black, they are red algae. Aquasabi attributes them to genera such as Audouinella and Rhodochorton [1]; both are genera of red algae (Rhodophyta) in the GBIF Backbone Taxonomy [4, 5].
Aquasabi gives a simple way to confirm the group: put a tuft in alcohol, and red algae turn reddish [1]. As with other aquarium algae names, “black beard algae” describes a look; which species grows in a particular tank is not something the sources cited here can settle.
Conditions sources associate with it
Aquasabi describes two different settings in which beard algae thrive [1]:
- Heavily stocked tanks with few plants. Here it links the algae to high organic load from overfeeding, infrequent water changes and debris built up in the filter and substrate.
- Planted tanks. Here it links them to an imbalance of trace elements, particularly iron, and recommends checking iron and keeping it around 0.05–0.1 mg/L. Its deficiency guide adds that over-fertilizing with iron may promote red algae such as beard and staghorn algae [3].
CO2. Aquasabi reports that it “is believed” that low free CO2 favors red algae such as beard algae, because they can take carbon from bicarbonate more easily than higher plants [1]. The wiki presents this as a belief, not an established finding, and this page does the same. EI Light’s advice when algae appear points in the same direction: remove the algae and infected leaves, then recheck CO2 before anything else [8].
Control
Sources describe three kinds of measure.
Fix the conditions. Large weekly water changes (Aquasabi suggests at least 50 %), cleaning the filter and substrate in heavily stocked tanks, checking iron dosing and CO2 in planted ones [1]. Tropica’s general algae measures, more algae eaters and more frequent water changes, apply too [7].
Remove what is there. Trim affected leaves and clean equipment. Aquasabi notes that blackouts, which work on several other algae, are almost totally ineffective against red algae [2].
Spot treatments. Aquasabi describes local treatment with carbon-based liquid products or hydrogen peroxide [1]. Seachem, maker of one such product, reports that hobbyists have observed less algae, beard algae in particular, when dosing Flourish Excel, but states plainly that it does not produce or market the product as an algicide, and that overdosing or applying it directly to plants can cause damage [6]. Any product used this way should be used according to its own label; see liquid carbon products.
Why it comes back
Removal does not change the conditions that let beard algae grow, which is why every source above pairs removal with water changes and a check of CO2 and nutrients. On slow plants with long-lived leaves, such as Anubias, established tufts can simply be cut off with the leaf.
Questions
How can I tell beard algae from other dark algae?
Aquasabi describes a simple test: red algae turn reddish when placed in alcohol, which shows they belong to the red algae [1].
Does a blackout kill black beard algae?
Aquasabi describes blackouts as almost totally ineffective against red algae [2].
Calculators for this
- CO2 from KH and pHEstimate dissolved CO2 in mg/L from carbonate hardness and pH, with a temperature-corrected constant, the usual chart shortcut, and the pH needed for a target.
- Water-change dilutionWhat a water change does to nitrate, phosphate, TDS or any dissolved level, the change needed to hit a target, and where weekly dosing settles.
Plants mentioned
Related guides
- FertilizersIron and trace elements: what chelates do and how to dose themWhy aquarium iron comes chelated, what EDTA, DTPA and EDDHA products declare, how iron deficiency looks, and the iron levels dosing methods aim for.
- CO2 & lightCO2 from KH and pH: the formula, the chart and where it breaksWhere the KH/pH/CO2 chart comes from, why its factor of 3 differs from the temperature-corrected 3.5, and the conditions under which both estimates fail.
- CO2 & lightLiquid carbon products: what the labels claim and what is establishedWhat Seachem Flourish Excel and Tropica Carbon Nutrition say about themselves, what their pages leave out, and the hazard data PubChem lists for glutaraldehyde.
- CO2 & lightPhotoperiod: how many hours a planted tank's light should runWhat growers and retailers recommend for daily lighting hours, how photoperiod and intensity add up to the daily light dose, and when shorter days are advised.
- Algae & problemsGreen spot algae: what it is and what sources say causes itHard green spots on glass and slow leaves: the likely organism, the conditions growers associate with it, the phosphate disagreement, and how it is removed.
Sources
- Black beard algae (BBA). Aquasabi (aquascaping wiki). Audouinella sp., Rhodochorton sp.; red algae, turn reddish in alcohol; organic pollution in fish tanks; CO2 deficiency and bicarbonate use (stated as belief); iron imbalance in planted tanks; suggested iron 0.05–0.1 mg/L; water changes; spot treatment. Accessed October 12, 2026.
- Black-out treatment. Aquasabi (aquascaping wiki). Almost totally inefficient against red algae. Accessed October 12, 2026.
- Deficiency symptoms in aquatic plants. Aquasabi (aquascaping wiki). Iron over-fertilization may promote red algae such as staghorn or beard algae. Accessed October 12, 2026.
- Audouinella Bory de Saint-Vincent, 1823 (GBIF Backbone Taxonomy record 7816564). GBIF Secretariat. Genus in Rhodophyta (red algae), Florideophyceae, Acrochaetiales. Accessed October 12, 2026.
- Rhodochorton Nägeli, 1862 (GBIF Backbone Taxonomy record 2664542). GBIF Secretariat. Genus in Rhodophyta, Acrochaetiales, Rhodochortonaceae. Accessed October 12, 2026.
- Flourish Excel. Seachem Laboratories. Hobbyists report less algae, beard algae in particular; not produced or marketed as an algicide; overdosing or direct application can damage plants. Accessed October 12, 2026.
- Algae control. Tropica Aquarium Plants. More algae eaters, water changes, light control. Accessed October 12, 2026.
- EI Light for those less techy folks. John N. and Tom Barr, The Barr Report. If algae arise: remove visible algae and infected leaves, recheck CO2, possibly adjust the lighting period. Accessed October 12, 2026.