Yesterday I was browsing thru MSN.com and I came across a link to one of the most realistic aquarium screen savers I have ever seen.
The website for this program is http://www.simaquarium.com
If you click on the above link you can see two screenshots on the front page of the site. They let you run a 14 day trial before it has to be registered.
Make sure you have the hardware and software requirements as with anything that is highly graphical it can put a serious load on a system with low graphics or processor captabilities.