We had a poll on January 30th, 2001. Nearly 6 years ago.
98% of 600 people polled indicated that they either had the video card to run the 3D version of the Aquarium or would buy one.
If we're really going to limit ourselves to a .99 cent graphics chip, I would keep 2 products going:
Marine Aquarium 2.x which is the 2D background
Marine Aquarium 3.x which is the 3D background
Every boxed version of 3.x would come with 2.x and a hardware tester as part of the installer and it would suggest which version of the Aquarium to install.
Either that or hide objects that the graphics card cannot display.