Redish coating

[stonefox]

There has been a remarkable reddish coating on the inside of the glas in one of my aquariums. Coming soon again when I removed it. What is it and what causes it and how to remove it? Se attached file.

[abornet]

Some algaes are reddish or maybe the stone in there has a lot of iron in it and it's iron precipitating out? I cant really tell from the picture but Does the red coating settle in just certain areas?

[stonefox]

No all over the glass. Some places more then others.

[Stalker]

diatoms.