Kirk Baucom originally wrote the ASCIIQuarium animation in Perl. His website, and the original ASCIIQuarium, with links to other ports can be found here: https://robobunny.com/projects/asciiquarium/html/. There’s also a freshmeat page for the Perl version of the program here: http://freshmeat.sourceforge.net/projects/asciiquarium
J. Sommer took this idea and created a Windows screensaver out of it. Unfortunately, the website where that was hosted went down several years ago (In case it comes back: http://remmos.j.perso.sfr.fr/). Before the site went down, I was able to contact the author and get the source.
I made a few modifications. Specifically, I added collision detection for the shark to show the fish splat animation from the original Perl version. I also added an ASCII diver and some other objects. The new screensaver is marked as version 1.3 so you can tell it apart from the original.
The updated screensaver executable for Windows can be downloaded here: