The short answer
Black beard algae (BBA) is a tough, dark-grey to black tufted algae that grows in fuzzy beards on leaf edges, hardscape, filter outlets and slow-growing plants. It’s one of the more stubborn types, and it’s driven mainly by unstable CO2, excess light, and a build-up of dissolved organics. Fix those conditions and it stops spreading; spot-treat what’s already there.
What causes it
BBA loves fluctuating CO2 and low water movement. In a CO2-injected tank, swinging levels are the classic trigger; in a low-tech tank, poor flow and organic build-up do the same. Older, overgrown plants and dirty filters that leak organics feed it too. Strong or long lighting over an unbalanced tank makes everything worse.
How to remove it
- Improve flow and CO2 stability: aim for steady CO2 all photoperiod and make sure current reaches every corner. See our CO2 systems hub and CO2 for beginners.
- Cut light: shorten the photoperiod and reduce intensity — check the lighting hub.
- Spot-treat with liquid carbon: with the pump briefly off, dose liquid carbon (glutaraldehyde) directly onto affected spots with a syringe. Treated BBA turns red/pink over a few days, then dies and can be removed.
- Remove organics: more frequent water changes and cleaning the filter reduce the food supply.
Keep it gone
Once conditions are stable, remove badly affected leaves and let healthy plants take over. Consider Siamese algae eaters as ongoing helpers. For the full multi-algae strategy, see how to get rid of aquarium algae, and to keep plants strong enough to compete, review your fertilizer routine.