There are many aquarium style screensavers for the Mac but few are as realistic as Aquazone Bass Edition.
The accuracy of the fish and the movements they make are so real you won't believe it and if you get bored with one set of fish, you can change to another. In the preferences, you have the choice of Rainbow Trout, Blackbass and Blue Gill amongst others. You can also change the water type from Bubbling to Lake which has more natural coral and nature although not enough for my liking. In addition, you can use the mouse as a hand to tap and tickle the fish. The fish seem to respond although usually only to turn around and wriggle a bit.
Aquazone Bass Edition is one of the most realistic aquarium screensavers you will find on the Mac although water environments could do with spicing-up a bit.