If you can feel the scratch with you nail, you're out of luck. For light scratches, you can use the sanding paper polishing technique. You can sometimes find glass polishing kits for removing scratches. Be sure that it's an abrasive based kit rather than chemical as some of the chemicals can be nasty.
Try filling the tank with water first to see what the scratches look like once it's full. Many scratches disappear once the tank is full.
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