Get some algae eaters such as Otocinclus they stay small or Siamese Algae Eaters, nether of them will hurt you plants and will do a good job at controlling the algae.
vonrampe, there are a few types of algae that are greenish, and algae eaters don't eat all of them. The slimy algae that covers like a film that Peter is asking about is not technically an algae even but a bacteria... and no fish will eat this. So its best to search the net for pics of algae to see what u have, to best treat it properly. Lighting, overfeeding, or water quality could all lead to algae, so as a fish will definitely help, it might be easier to cut back some of these other things to help solve your problem. Just a thought.
