Is the tank glass or acrylic? If acrylic, it's a matter of using a combination of very fine sandpaper along with a product from Novus that's a liquid scratch remover. See here
. I've used Novus #2. There are others there that may work as well. You can sometimes get these at a local home improvement store that sells plexiglas. Auto parts stores would have the very fine (wet/dry) sandpaper. If you find scratch removal 'kits', that's pretty much what they consist of. They just charge more and don' t give enough of what you need.
You start with about a 300 grit sandpaper sanding in a circular motion around and over the scratch. Use it wet. Work down to about a 1500 grit, and then use the Novus #2. Works pretty well. Just takes some time and patience. Start with a small scratch and practice.
If the scratches are more like deep gouges, it'll still work, but because of the amount of material removed, there will be some optical distortion.
If the tank is glass, I hear there are similar kits, but it's much harder to remove scratches from glass than acrylic.