While acrylic aquariums may scratch more easily than glass, the scratches can be removed by sanding and buffing, unlike glass. Scratches and scuffing can also be removed from both the inside and outside of the tank using products made specifically for this purpose. Here's how.
Empty the tank of fish and water. If you are removing scratches on the inside, it produces particles of acrylic through sanding that may not be good for your fish.
Sand in one direction only with 120 or 220 grit wet/dry sandpaper for deep grooves, 320 grit for scratches that can be felt with a fingernail, 1800 or 400 grit for fine scratches, 2400 or 3200 for light scuff marks, or 4000 or 6000 grit for very fine scratches.
After the first sanding, move to the next grade of sandpaper and switch sanding direction.
Continue working through the grades of sanding until you reach the finest grit. Switch direction with each grit. Never work in a circular motion, which can cause swirls on the acrylic surface.
Expand the area you are sanding by about 4 inches with each grade to avoid distortion from sanding down one small point.
Clean the aquarium with clean water and a drop of detergent once the surface has been restored to your satisfaction. Use flannel for cleaning and drying, and avoid paper towels.
Abrasive liquid cleaners like Novus Buffing Compound are available from aquarium supply stores.
Do not use ammonia or window cleaner on acrylic.