I recently got a used 75g tank. When I got it I cleaned up the tank and got it set up. However now that water is in it I noticed that the top inch of the tank has a white tint to it. It looks like it is a faint line or mineral deposit. I have tried scraping it with a razor blade scraper but with no luck. I would really like to get rid of this line but I don't want to drain the tank, I just got it going less than a week ago. Any tips for getting this stuff off?