Black algae can leave nasty black spots on your aquarium glass so treating it effectively and immediately is important. Algae needs plenty of water, light, and nutrients and if your aquarium has a lot of algae, then your water is getting lots of light and has lots of nutrients dissolved in it. There are ways to try and reduce and eliminate black algae.
Reduce the nutrients in the water by changing a portion of water in your aquarium each day until it lightens. Do not change all the water at once. Change only 20% a day.
Add fish to eat the algae. The Plecostomus Catfish and the Siamese Algae Eaters are both aquarium fishes that will eat some types of algae. The fish may not eat all of the algae but they may help to eliminate some of your algae problems.
Reduce the amount of light in your aquarium. Move your aquarium away from windows and try to keep out of direct light. If you cannot move your aquarium try wrapping it in aluminium foil where the sun hits it.