A buffer with a soft pad and automotive rubbing compound. There are 3 or 4 threads on here where people buffed scratches out of their acrylic tanks. You can buy the paste at auto-zone, most people use it to remove the haze from headlight covers.
Its a long process, with lots of work and sore arms, even with a high speed buffer, but you can make it look almost new. Search the forums here, there are several with before/after pictures.