What it looks like
Dark tufts 3–5 mm in size—from grayish-green to almost black—sitting in dense clumps along the edges of leaves, on driftwood, rocks, and filter tubes. They feel stiff and are difficult to remove, often coming off with a piece of the leaf.
Colloquially, it's called "black beard algae" or "BBA". It's a red algae, and its dark color comes from pigment, not dirt: when removed from water and dried, it turns red.
Why it appears
Three reasons, usually in combination:
CO₂ fluctuations. Not a low level itself, but an unstable one: CO₂ injection turns on late, the cylinder is empty, the diffuser is clogged. Plants slow down during fluctuations, while this algae tolerates them well.
Strong current in one spot. It settles where the flow is constant: filter output, a narrow space between rocks, the edge of a leaf positioned across the current. This is how it's recognized—it doesn't grow throughout the entire aquarium, but in streaks.
Organic matter. Infrequent water changes, untreated driftwood, a layer of sludge in the substrate.
What to do
It cannot be removed mechanically: fragments will regrow.
- Stabilize CO₂, if present: a constant level is more important than a high one. For an aquarium without CO₂ injection—reduce lighting, otherwise plants won't keep up.
- Spot treat what has grown. Turn off the pump, draw 3% hydrogen peroxide into a syringe, and squirt it directly onto the tufts in the water, 1-2 ml per spot; after 10-15 minutes, turn the filter back on. Within one to two days, the treated algae turns red, then whitens and detaches. The total dose should not exceed 20 ml per 100 liters at a time, and no more than twice a week.
- Driftwood and rocks are easier to remove and treat in a basin, rather than in the aquarium.
- Leaves completely overgrown are best trimmed: there won't be any clean spots left on them.
Caution with invertebrates
Shrimp and snails tolerate peroxide significantly worse than fish. For spot treatment, you don't need to remove them, but avoid pouring near a shrimp colony: gently push them away from the treatment area with a stream of water and do not exceed the dose. If shrimp are the main inhabitants of the aquarium, it's better to stick to mechanical removal and CO₂ stabilization.
What not to do
Do not pour peroxide into the entire tank volume. Spot treatment targets the algae tuft; pouring it into the aquarium harms filter bacteria and fish. This is not the same method at a different dose; it's a different approach.
Do not rely on fish. Adult black beard algae is only eaten by Siamese algae eaters, and even then, not all of them and not always: once they've tasted fish food, they lose interest in it. As a sole solution, it doesn't work.