I found a very interesting atricles about this common (?) menace. http://naturalaquariums.com/plantedtank/0608.html http://freshaquarium.about.com/cs/maint ... egreen.htm Basically, what aquarists call "blue green algae" is actually a type of bacteria organism called "organism cyanobacteria". But I suggets you read the articles before I get nailed for plagarism (and i don't have time to re-write it now!!) cheers.