Glass mirrors have a coating of aluminum on the back face, covered with a protective coating. You could scrape off the protective coating, but it would take an acid to etch off the aluminum and do a good job of it.
I had been considering using a mirror background for my tank, but if you are willing to do a lot of work to get rid of a good mirror, I think I will pass on my idea. When I got the tank the original owner had an aluminized mylar background on it, which looked very bad to me, so I pulled it off, but I figured it looked bad because the mirror was so imperfect. Is that what bothers you about the mirror?