Black Brush Algae is a form of red algae (Rhodophytes) in the genus Audouinella. While most Red Algae only thrive in saltwater conditions, a few of them have adapted to freshwater conditions and the Black Brush Algae is one of them.
The Black Brush Algae does have a photosynthesizing auxiliary pigment called phycoerythrin which absorb blue light and reflect red. For that reason, Black Brush Algae is usually black to purplish-gray instead of green like other algae. It forms short hairs (0.75cm long) that grows closely packed together. It grows on slow growing plants and solid surfaces such as decoration and substrate. When growing on plants, in usually grow around the edges of the leaves.
Unstable or too low Co2 levels seem to be ideal for the black brush algae to grow.
An imbalance in nutrients will also trigger this problem.
Oxygen: Can be successfully treated with a syringe of hydrogen peroxide. Overdosing peroxide will harm/kill your fish so I recommend no more than 1 ml of peroxide per 5 gallon. It works well but can’t be considered as an efficient treatment.
Co2: It’s been noticed that red algae are especially troubling in waters with plentiful CO2. Increase levels slowly to 30ppm will help lot. Try to keep Co2 levels as stable as possible.
Overdosing Flourish Excel can be used as an alternative to CO2.
Fish: Few fish will eat red algae. Siamese Algae Eaters and the American Flag Fish are known to eat it. Please let me know if you know other fish that eats Black Brush Algae.
Nutrients: In planted aquarium, the best way to eliminate Black Brush Algae is to let the plants out-compete the algae for the nutrients. Balanced nutrient levels, stable Co2 and enough light for plants growth are a must.
In non planted aquarium, regular water changes will help reducing nutrient levels.