I'm in aviation and was about to recommend the Micro-mesh kit that we use to buff wing tip lenses and the round windows everyone looks out of when flying. They are all acrylic and the kit works on them like magic. I've handsanded acrylic windows and wingtip lenses that were FAR worse than anything you will EVER do to your fish tank. They were just like new when I finished and I didn't even use the liquid polish that the kit supplies.
I now have an acrylic tank and have earned a few scratches from sand kicking up and getting stuck under the magnet when I cleaned it. I plan on cleaning up the scratches next week with the use of the same kit. I plan on using the smaller cleaning magnet from my old tank and wrapping the sanpaper around it then securing that with a rubber band. Also use a piece of felt cut to size for the outside portion of the magnet which helps it glide easier on the acrylic.
I have no fear of screwing up the tank and since the sandpaper in the kit is a wet/dry paper, and is designed primarily for wet sanding, it should go very fast. The trick is to know how to sand properly and having the eye for when the scratches are out. Just take your time and work through each grit, don't skip one to speed it up. Whn you are done you'll be very happy with the results.
Good Luck
