I've had to take scratches out of a large acrylic tank before without draining it. You use one of the scratch removal kits like the one form rainbow lifeguard. Grab a snorkel and a cheap mask. It can be a long process depending on the size of the scratch but it will come out. You won't need to do any curls wrist or bicep for a long time after.
Now if this where a glass tank you'd be screwed as you would have to use diamond paste for the final buff.
Now if this where a glass tank you'd be screwed as you would have to use diamond paste for the final buff.

