This is a nice site that has some info about our BGA, Oscillatoria.

Oscillatoria

As mentioned, it forms a nice benthic mat.
I've found this species common in FW springs seasonally, generally where there is a lack of plants or disturbed sediments.

The flow is high at such sites, so that can be ruled out as well as nutrient/water quality related issues, they are stable and have been for hundred's of years.

Seems several things allow them to grow, generally organic matter, unstable sediments, low light(but not very low).

In our tanks, low NO3 can indice them.
In the FW spring, the NO3 was about 0.2ppm.
But is replentished constantly due to flowing underground spring water, lots of plants there, and CO2 rich water(30ppm).

Regards,
Tom Barr
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I've noticed that it often grows in my tanks on the outflow of my hob's...could this be because that water is more richly oxygenated? The tank can be totally bga free but it will always grow there on the filter to some degree, for some reason.