I have a 100G tank with red spots on the glass only?

I have a 100G fish tank with African Sicklets it has been healthy for around 2years now. the fish are breeding and all my tests are normal ranges. In the last 2-3 weeks I have noticed redish-brown spots that have appeared only on the glass. I can scrub these off with the brush but it requires alot of force. what can cause these and how do I prevent it from happening again?