You can polish a scratch out of glass, but it may be more work than it's worth. Several companies make a polishing compound to polish scratches out of glass, mostly aimed at car restoration enthusiasts. You would most definitely need to drain your tank, and wash it VERY thoroughly after polishing the glass though, which is also going to require a little bit of skill and patience to do right. If you are still interested in possibly attempting to polish the scratch out, here is a link. This site is in no way competition to SWF, unless they are getting into the car restoration business, so I hope that the mods find no offense in posting a link.
www.eastwoodco.com and just do a search for "glass polishing".
HTH
Jeremy