The glass cover of my tank used to be very clear and see through, but over time it has become covered by some kind of stain, I'm pretty sure it is calcium or lime deposits. I have tried scrapping it of with a blade, I used steel wool, I sooked the cover in vinegar, I even used windex. Nothing works. I'm starting to get worried that my plants will not get all the light they need. Could anybody please give me some advice? I would appreciate it very much.