How about this:
Allow a video cam to give input into the aquarium.
This was mentioned by someone on another topic but I don't think people read it. At first is sounds crazy but there are already a few screen savers written doing it. They came with my video cam! The API is called Reality Fusion. Imagin the fish swimming up to the top of the screen when you walk in. This would be great!
Info from Intel:
       
ftp://download.intel.com/software/id..._Libraries.pdf
Info about the SDK:
       
www.seesaw.com/corp/Products/sdk
This is an example of a program written by a kid:
       
www.ews.uiuc.edu/~awu/pix...light.html
-Bill