I wouldn't buy a fisheye. Just buy a 20mm or something like that, it will give you, maybe 120 degrees rather than 180, and it will be way cheaper. Check on ebay for second hand or old lenses that fits on your mount. Old lenses, in most cases are just as good as the new one they put out.
Attachment are shit.
100$ for lenses is nothing, and if you want a fish eye or 20mm prime, its like 5-6 times more so save money!
I'd rather get a super bright 50mm f.1.4 or something... cheaper and plenty of angle of view.