That might be etching on the glass. I think when the water evaporates from the tank then condenses on the glass it actually dissolves the glass because it is distilled water. If you search the APD mailing list archives this topic was discussed recently and one suggested remedy was a liquid glass polishing kit available online. Some reported they tried physical polishing with no real benefit. The APD archives are here:
http://fins.actwin.com/aquatic-plants/
The thread was in the last month or two.
Tony