i would try a 3M automotive headlight polishing kit (may take more than one)...it goes on a drill, comes with a disc and a variety of grits of sandpaper (down to rediculously fine) and a foam buffing pad and buffing compound to polish the plastic back to clear...its about a 4 inch pad so it will be tedious (after all its made to do 2 headlights) but at least it gives you all the grits and steps where you could go to a body shop supply and buy larger versions of the same stuff and more compound if needed...i've used one to restore the shine on a 25 year old acrilic jacuzzi
If you end up placing black acrylic in the tank, I wouldn't use silicone. There are glues made for the purpose that work much better and set up almost instantly. Look up Weld-On #3 for example.
Update. Acrylic sheets came. Tank looks VERY nice.
Here’s my concern: The bottom is making this flap and I think some fish could fit and possibly get stuck? Tank is running with water. How can I best attach the poster to the back? I would like a method that won’t require me to break down my rocks please.