On the inside or outside? If it's on the inside I use a razor blade. On the outside I just use Windex wipes. I have noticed that back when I used Prime declorinator it made streaks that were near impossible to clean off if I accidently dribbled some on the outside glass. Do you use Prime, by any chance? After I went back to API declorinator I didn't have any more problems. I always clean the glass with the Windex wipes as soon as any water gets dripped on it. The longer it stays in contact with the glass the more time the calcium deposits have to stick to it.