What kind of algae?

[PostalPenguin]

I have something "growing" in a few spots on my 20g. It looks like a hard water stain but its on the inside of the glass and it slowly grows over time. Its a slight purplish to brown color. It's not likely diatoms since my oto keeps the tank clear of them but it is also rather difficult to remove unlike diatoms. I have to really scrub it with a paper towel to remove it.

Any ideas? It reminds me of the coraline algae that you see in reef tanks. Its not any of the brush or hair algaes but I dont think its cyano either since again its quite hard to remove.

[Anomaly]

I have something "growing" in a few spots on my 20g. It looks like a hard water stain but its on the inside of the glass and it slowly grows over time. Its a slight purplish to brown color. It's not likely diatoms since my oto keeps the tank clear of them but it is also rather difficult to remove unlike diatoms. I have to really scrub it with a paper towel to remove it.

Any ideas? It reminds me of the coraline algae that you see in reef tanks. Its not any of the brush or hair algaes but I dont think its cyano either since again its quite hard to remove.

Check out the first post in this link:

http://www.wetwebmedia.com/FWsubwebindex/FWAlgID.htm

Sounds like it is possible to have coraline algae growing in FW tanks.