Who knows if fish really see the world the way a fisheye lens does. The good news for them if they do? They can see a lot of things coming for them. Fisheyes are known for extremely wide angles of view, and extreme distortion of space. Don't think this is a bad thing: when used in the right situation, fisheye lenses can lead you to new and unique perceptions of the nature of spaces around you. Try a lens like this on a city street, or a long subway escalator, or taking a close-up of your dog. You will no doubt find new and different ways to document the world around you.
A circular fisheye gets most all of what's in front of you and puts it into a circular image surrounded by a black field. If you've ever seen a convex mirror, the effect is very similar. This Sigma Photo 8mm circular fisheye features an f/3.5 aperture.
Limited use with APS-C sensor cameras
While this lens will mount on an APS-C sensor camera, your view will be cropped to the center of the lens's projection circle of light, so much of the characteristic barrel distortion you expect from a fisheye lens will be outside the sensor's field of view. Should you use it on an APS-C camera, you will have a somewhat distorted wide-angle image at an effective focal length of 12mm on most cameras with sensors that size.