I've always thought aquariums as screensavers were a no-brainer. Most are lacking, limiting themselves to small fish you'd find in just about any aquarium, freshwater or salt water. MyReef 3D Aquarium [iTunes Link] isn't going to shake up the genre, but it is certainly the most realistic, smoothest and most relaxing "fish simulator" I've used so far. It serves no purpose other than to completely mimic the function of a real aquarium: to sit there and look pretty and be relaxing. And it does a good job.
You can add many types of fish, some aquarium decorations and plants, and feed the fish or tap on the glass. You can pinch to zoom in, seeing quite pretty 3D fish. These are not 2D sprites flopping around a flat screen, these are real, OpenGL graphics and they look terrific. In fact, something I love about this app is the attention to detail. Fish behave as their species would behave, including group behaviors and dorsal fin indicators. Those small touches will really appeal to any fish fans out there -- and there are plenty.
If you're an aquarium hobbyist you will love this app. If you want a pretty thing to look at or mess around with or relax to, MyReef 3D Aquarium is quit nice. It doesn't "do" a lot, but what it does it does well. I think it's a fair value at $2.99.