When I bought my used acrylic tank. I soaked it in vinegar and water to get rid of the coralline.
then I sanded with #400, #600, #1000, #2000
Then Novis 3, 2, 1,
Its important to remove All scratches from the previous grit paper, or the scratches will never disappear. I recommend sanding in one direction with the 400, then clean up all the residue before going to the 600. Then sand in one direction perpendicular to the direction of the previous grit. When all the scratches from the previous grit are gone. Clean, then repeat the process with the next grit. and so on till you are done. Cleaning between grits is important because if you get say a piece of #400 in while you're sanding #600, it will scratch the tank.
Its not an easy process, but do it right and your tank will look brand new.
you can see before and after pictures of my tank here
http://www.chicagoreefs.com/forums/showthread.php/18811-My-250
Good Luck and Happy Sanding,
Rick