Live aquarium plants are a great way to decorate your aquarium while providing naturalistic hiding places for fish. Aquarium plants also help keep the water clean. However, they are vulnerable to different types of algae growth, which looks like black sludge on the leaves. Black stuff on aquarium plants is difficult to treat because they cannot be scrubbed clean like fake plants, but treatment is possible.
Some types of green algae can have a very dark, almost black appearance and grow on plants, decorations and walls of the aquarium. These types of green algae include green spot algae and pelt algae, both of which adhere to the leaves of the plants. Both of these types of algae can be treated by increasing the number of regular water changes in the aquarium as well as adding algae-eating aquarium creatures like snails and plecos.
Black spot algae is a type of red algae species. Black spot algae grows best when there is plenty of light and nitrates in the water, and it is difficult to remove. In most cases, plants covered in black spot algae will require removal from the aquarium.
Blue-green algae is found in swimming pools and aquariums. It has a very dark, black-colored appearance and can completely coat plants. Some snails will eat blue-green algae, but even algae-eating fish won't. Increasing water changes and leaving lights off the aquarium for a few days can kill blue-green algae, after which you can be remove it from the tank.
Diatoms are brownish-black slimy growths that appear on aquarium plants and throughout the tank. These growths can be combated with more lighting, lower pH and by adding algae-eating fish or snails.