Here is a website with the midi version of "The Aquarium", as well as many others... however there is a "2 midi per day" limit.
http://kunstderfuge.com/saintsaens.htm
I love the Shedd Aquarium, I've been there a couple times. I'll never forget the seahorse special exhibit there once.
I like the idea of letting the screensaver play a selection of your music that you choose off your hard drive, that's pretty awesome.
Rebecca