Black beard algae on the slow leaves
It started as a few dark tufts on the edge of the oldest anubias leaf, the kind you can rub between finger and thumb and feel as bristle. I ignored it. Six weeks later the leaf edges of the sword plant, the heater, and the filter outlet were all wearing it. Black beard algae likes old leaves, bright light, and water that does not move much, and I had been giving it all three.
What I tried first did nothing: a three-day blackout with a towel over the tank. The green algae died, the black beard did not care. Then I did three things at once, which is bad science but I wanted the tank back.
First, light. The photoperiod went from nine hours to six, and I moved the light up one notch on the bracket, about 5 cm, which the plants tolerated. Second, flow. I turned the filter outlet to push across the back wall instead of straight down, so the dead corner behind the sword plant finally had movement. Third, spot treatment. I pulled the anubias out (it was on a rock, so that was easy) and dabbed the affected leaves with 3 percent hydrogen peroxide using a cotton bud, left it for two minutes, and rinsed in a bowl of tank water. For the sword plant, which was rooted, I turned the filter off, lowered the water 10 cm, and squirted about 2 ml of peroxide directly onto the worst leaves with a syringe, waited three minutes, and refilled.
The treated tufts went pink, then white, then gray over four days, and the shrimp cleaned them off. I repeated the syringe treatment twice more, a week apart, cutting away the two oldest sword leaves that were more algae than leaf. The shrimp and tetras showed no reaction to the peroxide at those amounts; I would not go above 1 ml per 10 liters in a tank with livestock.
Three months on there are still a few bristles on the heater, and I have decided to live with them. The sword plant has eight clean leaves. The real lesson is the dead corner: once the water moved there, nothing new grew.