Is a Nikon D3100 compatible with a Opteka 6.5mm Fisheye?

Just curious as I'm considering saving up for them and I want to be sure that they're compatible.
If not, could you list some alternatives?

Opteka is a third party lens… They make different versions of their lenses for the different brand cameras. You would need to make sure you got the version specified for Nikon mounts. Also note, this lens does not have an autofocus motor in it, so your D3100 will not focus, you will have to focus manually.

If the Opteka has a Nikon lens mount, sure it will work.