Logically, people use it a screen saver or Aquarium simulator, and it "defeats the purpose" if the Aquarium is a narrow image on a mostly black screen.
The Aquarium provides you the options to run the Aquarium in a window. You can hide the title bar of that window. You can also "lock the camera" of the Aquarium to any desired section of the Aquarium.
This is not like a widescreen film where the director's intent is for you to see the entire image at one time. Jim designed Marine Aquarium 3 to allow you to move the camera to your favorite part of the tank and leave it there, or set it to auto-pan. If you want to see the entire Aquarium at all times, that's possible with two 16:9 widescreen displays.
The number of users who would choose to run the Aquarium as a thin ribbon (on a 1600 x 1200 screen, it would use just 1600 x 450 of the display with 2/3 of the screen black at all times) could be counted on one hand. Yet we'd probably receive several tech support e-mails per month from users who accidentally enabled this feature.