Marine Aquarium is a program designed for Mac that displays a fish tank on your desktop. The application provides you with accurate information about various fish species, gives you access to many display and sound settings, and shows a calendar and a clock in the background.
You can design your own tank by adding as many fish as you want and changing the background picture. The app even allows you to import your own music files and plays them as soon as you launch Marine Aquarium on your desktop (it imports music from your iTunes library).
Another good thing is that this program comes with multi-monitor support. Plus, you can resize the main window of the utility according to your requirements.
The one aspect I don't like about this application is the fact it uses a large amount of CPU resources while it's running.
As for the rest, Marine Aquarium is a nice looking and simple to use screensaver utility. And since it comes with a reasonable price I can't find any reason why you shouldn't try it on your Mac.