My monitor is a Super Ultrawide at 49in Wide for the screen set at 5120x1440.
Crazy!
I think it explains the cut-off/panning issue. Your desktop is wider than the aquarium!
When this was made I'm guessing a large desktop was 1024 wide and dual screens were rare, so the background size was more than enough. You're at 5 screens wide and I don't think anyone expected that back then.