the black ring around your movie, is called vinnetting
doesnt happen with all lenses, its a combination of the lens and the camera. and if you want to you can zoom though the vinnetting, but when ever you zoom in with a fisheye or a wide angle lens you loose quality, it may not look like it at all when using you screen, but if you compare the footage though a monitor you will be able to tell the difference.
and with any cheap fisheye you lose quality from your footage. and the edges of you film will be blurry depending on what lens you get.
how much are you willing to spend on a lens?
if you are looking for a cheap on kenko has a 43mm one for around 40$ and you could just buy a step up ring to fit it.
and there is raynox qc 303 which is also a cheap lens, has more distorsion than the kenko (.03) but you lose much more quility.