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Dream Aquarium is a next-generation aquarium screensaver that brings the beauty of a lush freshwater aquarium to your computer with unsurpassed realism.
On the product site, they have a free demo download and a Quicktime movie that:
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shows off the amazingly realistic behavior and motion of Dream Aquarium's inhabitants. These are not real fish! They are 100% computer generated fish doing everything that real ones do - pecking at objects in the tank, eating food, chasing each other, swimming amongst the plants, hanging out in schools and enjoying life!


http://www.dreamaquarium.com/
The fish and scenery are not too cartoon-ish. I find myself sitting and watching this thing with my (mock-up) projector. It's quite relaxing. I can't wait until they add more features.