Random orbital sander... no need to change sanding directions. Dont do anything until you go to harbor freight and buy a multi speed polisher. I think it's safe to say that you will never polish that big of a tank by hand.
You need a ROS, 400,800,1200, 1500, 2000, and 5000 grit sandpaper, a spray bottle with water and a drop of dish soap, your polisher, and novus 2 (the light scratch remover).
Find the big scratches on the inside and mark them with tape on the outside. Start with 400 grit and use even pressure. Back and forth back and forth until the scratch is completely gone. Move on to the next scratch, do the same.
Now you need to remove the scratches from the sander. Spray all the areas you sanded and wipe with s cloth!!! (The grit from the paper falls off and you dont want to sand with 800 with bits of 400 on the panel). Sand with the 800 and slightly enlarge the area to feather it into the areas you didnt sand. Spray clean, then 1200, enlarging the area. Each successive grit will take longer because you have less cutting power and you have to sand through the precious grits scratches. When you get to the 2000 grit keep the panel wet while sanding. You can likely polish all the remaining scratches out after 5k but honestly I find it easier to wet sand with 10k then polish.
Amazon has the really high grits. Make life easy on yourself and flip and turn the tank so you are always sanding down... not trying to sand on a vertical plane.
Polishing is straight forward, use a low to medium speed. Reapply polish when you can no longer see it swirling around.
Then just polish the outside.
It will likely take 8 hours to sand and polish. If you go up to 10k before you polish I PROMISE that the tank will look even better than new.
Trying to take deep scratches out with polishing only has a higher chance of creating distortions because people tend to turn the polisher on its edge and go to town on the scratches.
While it may seem frustrating and you will curse acrylic, this is actually the beauty of acrylic.. you can fix scratches!!!