Okay, I had a bit of that at the very top of my tank. I think the glass might be etched. For me it was okay because once I filled it up with water, I couldn't see the etched bit anymore. One thing you could try(in case it's not actually etched) is a product called CLR. It removes calcium, lime and rust but it's something I didn't want to recommend as you have to make sure it is totally rinsed out. It does claim to be safe for the environment but you have to wear gloves.