Algae can be useful in your aquarium, in small amounts. Large amounts of algae can turn your water green and make your aquarium look awful. It can also clog your filters. Controlling this green monster is easy, if you catch it early.
A small amount of algae in your aquarium may be helpful, but large amounts can cause problems. Besides being ugly, it can clog your aquarium filter system and cause Ammonia and Nitrate Spikes. Dead algae can be an even more serious problem because it rots and puts ammonia into your water in large quantities.
If your aquarium is near a window or you leave your lights on all the time your algae growth can get out of control. This can happen gradually or all of a sudden.
Controlling algae growth and removing the dead algae from your aquarium system can take a few days. You may be in a situation where the amount of dead rotting algae that has dissolved in you water is causing high ammonia levels. If this happens, you should treat for an Ammonia Spike.
If you get a sudden bloom of algae growth on your aquarium glass, decor, and aquarium equipment, possibly along with green water, then you may have dissolved biological matter and nutrients in your water. This along with sunlight or excessive aquarium lighting will cause an explosion in algae growth.
Try to avoid using algaecide to control your aquarium's algae problem. It can be harmful to sensitive fish. It also causes a mass die off of algae and increases the chances of having a serious increase in the amount of ammonia in your aquarium's water.
Adding aquatic plants to your aquarium after you have solved this problem will help you keep algae under control. Aquatic plants compete with algae for nutrients in the water.
For more information, go to Wikipedia's Algae page.