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It is not calcium deposits. It is etching of the glass, itself, by the water over a long period of time. Glass is a wonderful product, but it is slightly soluble in water. I have some old tanks where the glass can be seen to be slightly cloudy when dry. When filled with water the glass still looks clear. However, at the tops of these tanks, there is a lot more etching and depositing of silica by the water line, which moves up and down, leaving about an inch and a half of more seriously clouded glass. There are polishing compounds that can be used to polish the glass, but you need an electric drill equipped with a polishing attachment. Progress is agonizingly slow if you try to do it by hand.
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