Pure Evil
Nicked from G.F
Cyanobacteria (BGA)
Strictly speaking this is not a species of algae but a form of bacteria. It does, however, act in a similar fashion to algae and can be prevented using similar principles.
I am no expert on BGA (or algae for that matter) but I can share my experiences with it in the hope that it may help you.
Also known as Slime Algae, due to its physical nature, it covers the substrate and plants with a blue/green slime like substance. Untreated this will spread very quickly smothering everything, substrate, plants and décor. It out competes the plants for nitrogen and prevents photosynthesis due to it blocking the plants from the light. It is also toxic to fish and nothing will eat it. In short it is nasty stuff – it even has an unpleasant smell. This is one method of confirming it is BGA.
I understand it originates in the substrate, this is certainly true in the couple of cases I have had. I am unsure as to what the true cause of its manifestation but I think stagnant water helps it and the anaerobic bacteria associated with it. Low nitrates and natural light are often associated with its spread too.