Algae is usually caused by an excess of nutrients(overfeeding, messy fish, infrequent water changes and substrate vacuums) and light(especially daylight from a window). As for are they good for the aquarium... to me some algae looks more natural. It also depends on what kind of algae. Some(like cyanobacteria or blue/green algae) are indicators of water quality problems. Others are good if you have fish that need algae in their diet. Heres a good article on algae
http://www.plantgeek.net/article_viewer.php?id=9
Carol
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