Black Beard Algae (Audouinella): How to Stop It and Keep It Away
Description
Black beard algae is not entirely black: it forms tufts of dark grey, almost black filaments, hard to the touch, on leaf edges, around the filter outlet and over rocks and wood. Up close it looks like a few millimetres of fuzz; with time it grows into one or two centimetre mats that grip whatever they touch.
It is a red alga of the genus Audouinella and it does not go away on its own. It favours hard-leaved, slow-growing plants such as anubias and Java fern, and tanks with injected CO2 where the gas level swings up and down.
Algae-eating fish and shrimp help with young beard, but they do not solve an outbreak: before scraping, fix the cause.
Why they appear
Unstable CO2
This is the most common cause. Injection that swings up and down during the day, a bottle that runs out, a badly placed diffuser or an overloaded reactor leave the plants short of CO2 at times, and black beard algae takes advantage of those gaps while the plants struggle.
Treatments
Stabilise the CO2
- Switch the CO2 on an hour before the light and off an hour before the light goes out.
- Adjust the bubble rate gradually and never overnight; follow the drop checker until it settles on lime green.
- Check that the diffuser spreads the gas evenly and that the cylinder does not run out mid-week.
Spread the flow more evenly
- Aim the filter outlet so that water reaches every corner.
- If a planted area barely moves, add a small pump or change the direction of the outlet.
Built-up organic waste
A dirty filter, pruning debris on the bottom, overfeeding and infrequent water changes raise the organic load. Black beard algae thrives in that loaded water, especially near the filter outlet.
Treatments
Water changes and cleaning
- Change between a third and a half of the water every week and vacuum the bottom.
- Rinse the filter media in water from the tank itself, not tap water, so you do not kill the bacteria.
- Remove dead leaves and pruning debris before they rot.
Feed less
Give only what the fish eat in a couple of minutes and remove whatever is left.
Weak or deficient plants
With few plants, or with leaves damaged by a lack of fertiliser, the alga finds room and nutrients that nobody contests. The worn edges of old leaves are its first anchoring point.
Treatments
Remove and spot-treat the affected area
- Trim heavily covered leaves: they will not recover.
- Take out the affected rocks and wood and scrub them with a brush out of the water.
- On tufts you cannot remove, apply liquid carbon (glutaraldehyde) with a syringe directly onto the alga, with the filter off for a few minutes. Stick to the manufacturer's dose: too much harms invertebrates and sensitive plants such as vallisneria.
Strengthen the plants
- Increase the density of fast-growing plants, which compete for nutrients.
- Fertilise with a complete macro and micronutrient fertiliser at the dose the tank needs.
Sources
- GBIFGBIF Secretariat (2026). GBIF Backbone Taxonomy. Checklist dataset. https://doi.org/10.15468/39omei. Consultado vía GBIF.org, 2026-08. Audouinella.
Taxonomy verified as ACCEPTED unless stated otherwise.
The scientific name is a genus (spp.): the whole genus is catalogued because its species cannot be told apart by eye in an aquarium.
The causes of appearance and the treatments have no citable source: GBIF supports the taxonomy, not the management of algae in aquariums. A source for algae is still to be agreed.