Black beard algae, also sometimes called black brush algae or simply BBA, are freshwater algae that bloom in water with high nitrogen and carbon dioxide concentrations. These algae, which carry the scientific name Audouinella, are in the Rydophyta algae family and bloom naturally in ponds, lakes, and slow-moving streams. They also grow in indoor aquariums and can be a particular nuisance for fish owners as they are extremely difficult to remove and eradicate.
Audouinella is a red alga variety. The “red” designation comes from the alga’s high concentration of the phycoerythrin pigment, which reflects red light. It absorbs blue light, which is one of the main reasons that black beard algae look, at first glance, almost black in color. These algae are typically much smaller than ocean-dwelling red algae. Their leaves form small, almost hair-like clusters, which give rise to the "beard" descriptor.
Black beard algae will only grow in fresh water, and even then usually only in water that is relatively still and exposed to regular light. They typically grow in clusters, and reproduce rapidly through sport dispersal. Most of the time, the algae attach to the leaves of other plants, but they can also attach to rock surfaces. In home aquariums, this alga is common on air hoses, crossbars, and most any other stationary, smooth surfaces.
There is nothing particularly offensive about the alga — it does not normally have much of an odor, and it does not steal nutrients from fish other sea plants. The problem comes with excessive growth. Black beards can cover entire aquarium surfaces and can effectively suffocate plants by blocking their access to light.
It is not known with any certainty what causes black beard algae to grow, particularly in home aquariums. The algae thrive in water that is rich in both nitrogen and carbon dioxide, however. Soft water seems to attract the algae more than hard water, as well. Most aquarium experts recommend regular chemical balancing, along with routine cleanings, to reduce the likelihood of a black beard bloom. Preventing black beard algae at the outset is significantly easier than removing them after the fact.
Getting rid of black beard algae in an aquarium is often a very difficult feat. The algae can be scraped away and the plants it is attached to removed, but if any spores remain in the water, a re-bloom is almost inevitable. Spores are impossible to spot with the naked eye and often persist on rocks and aquarium walls even after a thorough washing.
The algae can survive in even the harshest water conditions for weeks on end. They can usually also survive for extended periods of time without either sunlight or nutrients. Chemical treatments, particularly those designed to rob nitrates from the water, will sometimes work, but only over extended periods of time. If fish are sharing the aquarium with the offending algae, these sorts of treatments are not usually a viable option.
The Siamese algae eater is one of the only fish that will eat the algae. When added to an infested tank, these fish will usually feed on the algae. This can be a relatively stress-free way to return to a clean aquarium and is usually a less expensive alternative than starting from scratch.