Algae could be caused by two things: Excess nutrients and too much light. If you dont clean your tank well enough it could be the fish poo. And if your tank is in direct sunlight and doesnt have any live plants to combat the algae then you got a recepie for an algae invasion.

Ugh. It's BACK. It was gone for a week, and I really thought i got rid of it by boiling everything. I don't know why it came back. It always starts in the sponge of the filter. Should i just get a new filter? I don't want to have to scrub everything down and boil it AGAIN.