Are you sure its silicone? That would not stain the glass. Is it calcium deposits? Let the vinegar sit on the area for a long time. You can also try CLR Cleaner that works great.

For silicone you need a brand new razor blade to remove that very fine layer. You can also try more extreme measures like turpentine or naphtha soap. The tank would need a very good cleaning afterwords so try and limit the area you apply those to.