If it appears to be in "sheets", is quite slimy, gives off a strong
fishy or swampy odor and rapidly covers everything, including the sides,
it's caused by cyanobacteria, not common algae.
The best treatment is to first remove as much of the material as
possible by hand, cleaning it off the walls and plants, and removing as
much as possible from the gravel, ideally vacuuming it. Do a partial
water change, again removing as much of what may be free floating as
possible. Then treat the tank with 200 mg. of erythromycin phosphate per
10 gallons.