Black beard algae is the most common complaint in CO2 injected aquariums and the most frequently misdiagnosed. It is not caused by high light and it is not caused by phosphate. In almost every case it comes down to CO2 that fluctuates, organic waste that has been allowed to build up, or both together.
What is black beard algae?
Black beard algae, or BBA, is a red algae from the Rhodophyta group. Despite the name it appears black, dark grey or deep purple, growing in dense tufts two to five millimetres long that feel soft and velvety but grip like glue.
That grip is what makes it so frustrating. BBA anchors itself with a holdfast that embeds into the surface it grows on, so wiping achieves nothing. It favours surfaces that stay put: the leaves of Anubias, Bucephalandra and Java fern, filter outlets, spray bars, hardscape and CO2 diffusers.
One useful quirk. When BBA dies it turns pink, red or white before breaking down. That colour change is the pigment phycoerythrin degrading, and it is the clearest confirmation that whatever you did has worked.
Is it black beard algae or staghorn?
These two get confused constantly. They are closely related, they share the same causes, and they respond to the same treatment, so if you are unsure you can follow this guide either way.
What actually causes black beard algae?
Unstable CO2
This is the primary driver, and the word that matters is unstable rather than low. BBA is triggered by fluctuation far more than by any absolute concentration. An aquarium holding a steady 20ppm will usually stay clean. One swinging between 35ppm and 10ppm will grow BBA despite the higher average.
The most common version of this is not a broken regulator. It is injection that starts too late in the day. If your CO2 only reaches full concentration two or three hours after the lights come on, your plants spend the most demanding part of their day carbon starved, and that daily gap is all BBA needs.
pH drop from the degassed baseline across a photoperiod. Both aquariums reach the same concentration. The one on the dashed line spends three hours a day, every day, without the carbon its plants need.
The other common triggers are worth knowing because most of them creep up on you:
- A cylinder running down and emptying part way through the week
- DIY yeast and sugar systems, which produce wildly variable output as the mix ages
- A filter slowly clogging, reducing flow and with it CO2 distribution, over several weeks
- Plants growing in and blocking the flow path they used to receive
That last one catches people out constantly. Nothing about the equipment changed, but the aquarium changed around it.
Organic build-up
Dissolved and particulate organics feed BBA directly. Detritus trapped in the substrate, a filter that has not been serviced in six months, overfeeding, overstocking and decaying leaves left to break down all contribute.
Organics and CO2 instability frequently arrive together, because a neglected filter causes both at once. It carries less water, so distribution suffers, and it holds more waste, so organics climb.
Flow decides where it lands, not whether it appears
BBA is famously found on filter outlets and in high flow zones, which has led to the common claim that flow causes it. That is not quite right, and the distinction matters.
Flow does not create BBA. It determines where BBA settles, by delivering the highest volume of spores and organics to those surfaces. If your CO2 is stable and your organics are controlled, a high flow area will stay clean. Redirecting flow is a legitimate tactic, but on its own it just moves the problem somewhere else.
Slow growing plants
Anubias, Bucephalandra and Java fern hold the same leaf for a year or more, which gives BBA all the time it needs to establish. A fast growing stem plant sheds and replaces leaves long before that happens. This is why BBA appears on your Anubias while the Rotala two centimetres away stays spotless.
What does not cause black beard algae
Two myths worth putting to rest
- Light. Higher light makes existing BBA grow faster, so dimming will slow it down. It will not clear an infestation and low light will not prevent one. Plenty of low light aquariums have BBA.
- Phosphate. There is no established causal link between any single nutrient and BBA. Modern high tech aquariums routinely run phosphate above 2ppm with none at all. Cutting nutrients weakens your plants and hands the algae an advantage, so do not starve the tank to fight it.
How to get rid of it
Physical removal drops the biomass immediately and makes everything that follows more effective. Pinch and pull tufts off leaves with fingers or tweezers. Use a stiff stainless brush on rock and wood. Pull the outlet, spray bar or diffuser and scrub it outside the aquarium.
Never take a wire brush to a plant leaf. You will destroy the leaf and BBA will recolonise the damage.
For wood or rock you can lift out, an out of tank treatment is far more thorough than anything possible in the aquarium. Spray the wet surface with 3% hydrogen peroxide, leave it five minutes, then rinse thoroughly under running water before returning it. Rock can simply be boiled or scrubbed dry instead.
On Anubias and Bucephalandra a badly infested leaf will never look right again. Cut it off at the petiole, back to the rhizome. Rhizome plants regrow readily, and clean new growth beats a scarred old leaf.
Everything above removes the algae in front of you. None of it changes why the algae was there. If your CO2 still swings and your substrate is still full of detritus, you will be doing this again in a month.
How to stop it coming back
Stabilise your CO2
This is the whole game. Everything else is secondary.
- Target a 1.0 pH drop from your degassed baseline, reached by the time the lights come on rather than hours later
- Start injection 90 minutes to 2 hours before lights on to get there
- Confirm with a drop checker sitting green at lights on, not mid afternoon
- Keep a spare full cylinder. The gap between an empty cylinder and a refill is one of the most reliable BBA triggers there is
- Check your working pressure weekly. A cylinder running down shows in the bubble rate before it shows in your plants
A stable injection rate starts with the regulator. The AquaLabs CO2 range covers regulators, diffusers, drop checkers and check valves. To measure the pH drop properly rather than guessing at it, the AquaLabs Master Test Kit reads pH alongside ammonia, nitrite and nitrate.
Control organics
- Weekly water changes of 30% to 50%
- Siphon detritus from open substrate and around the base of hardscape
- Rinse mechanical filter media in old tank water every four to eight weeks
- Remove decaying leaves and trimmings rather than letting them break down in the water
Manage where the flow lands
Once CO2 and organics are sorted, redirecting flow is a useful finishing touch. Point the outlet away from Anubias, Bucephalandra and prominent hardscape, and along the front glass instead. A poppy style outflow disperses flow far more gently than a standard pipe and is a straightforward swap.
What eats black beard algae?
The Siamese algae eater is the only fish that will reliably eat living BBA, which makes it uniquely useful. A few things are worth knowing before you buy one:
- Make sure it is a true Crossocheilus. Chinese algae eaters and flying foxes are commonly mislabelled and neither will touch BBA
- Juveniles are far more effective. Siamese algae eaters become lazy grazers as they mature
- A well fed one eats less algae, so do not overfeed the aquarium if you want them working
- They reach 15cm and are active swimmers, so they need a tank of reasonable length
NQ Algae shrimp, neocaridina, otocinclus, bristlenose catfish and snails will all clean up BBA after it has died, and none of them will touch it while it is alive.
Be realistic about this. No clean up crew fixes a BBA problem on its own. They are excellent at removing the aftermath and useless as a cure.
The short version
- BBA is caused by CO2 that fluctuates and organics that build up, not by light or phosphate
- Remove what you can by hand, and treat removable hardscape with peroxide outside the tank
- Cut heavily infested Anubias and Bucephalandra leaves back to the rhizome
- Reach a 1.0 pH drop by lights on, not part way through the photoperiod
- Start injection 90 minutes to 2 hours before the lights
- Keep a spare cylinder so you never run empty mid week
- Weekly water changes, siphon detritus, service the filter before it clogs
- Redirect flow away from slow growing plants once the cause is fixed
- Add true Siamese algae eaters if you want help, but do not expect a cure from them
- Dead BBA turns pink or red before it breaks down
BBA has a reputation for being unbeatable, and it is not deserved. It is stubborn to remove and simple to prevent. Get the CO2 steady and the waste under control, and it stops being your problem.