You can actually polish acrylic while the tank is filled (I've done it), but you can't use a slurry as the final polish. However the "crystal bond"/"micro-mesh" polishing cloths can be had as fine as 12,000 grit (yes, 12 thousand). I've found you can usually stop at the 9,000 grit on a "wet" tank.
The Rainbow/Lifeguard kit has a selection of grit sizes. The key is to polish perpendicular to the scratches until the disappear, then when you go to the next finer grit, polish perpendicular to the scratches from the previous grit size until those scratches are gone...repeat this process until you're happy with the results. If you polish this way, you'll always know when to change grit size. Also, work in small areas as opposed to one large area.
HTH
What are you wondering about the foxface? I kept a Siganus vulpinus (no-spot) for several years, which is the same as the unimaculatus without the spot.