To cope with deep scratches in glass there are two approaches; I am unsure as to the aquarium saftey of either of these so, research, research...
1) As Sue says, you can regrind the surface using grinding paste. You would have to treat quite a large area to prevent weird optical effects. This would really be a massive DIY task. You may be able to find someone to do it professionally, but it would cost.
2) A more modern option is to treat it in the same way that chips are treated on your car windscreen. This involves filling the glass with a transparent resin and then polishing that level with your glass. This is still a big job.
Probably the only solution that makes sense for a DIY job is to turn your tank round so that the scratch is at the back.