Bear in mind the only thing that will be classified as an ultrawide on your 30D is a 10-22 or lens with similar focal length. (12mm is fine, too). You will want a 8mm fish (either Sigma fish for AF or Peleng fish for MF). Remember your crop factor.
I am an ex-10-22 owner who traded up to the 17-40. Before everybody starts thinking 17 is "ultra wide", it's only UW on full frame, and that's how I use it. I don't even consider it UW on a 1.3x crop!
Now that's out of the way these are my two favorite lenses on their respective platforms. More of my top favorite keepers have been shot with these than any other of my lenses. 
It depends though, whether you 'see' in ultrawide. And don't go crazy with the UW, sometimes a standard or a tele might do better.
A fish requires a special talent to use WELL. I have met one guy who was AWESOME with his full frame FILM camera, black and white film and his fish. Forget whether it was a 8mm or 15mm, I THINK it is 15mm because he is using FF and can get away with it. His work had the true mark of a fish expert, the way he positioned the objects to the side of the frame for maximum distortion while controlling the center so they 'framed' the subject was absolutely amazing. He has the eye.. I cannot 'see' in a fish eye so I do not use or own one.
He let me play with a Sigma 8mm as well, and while it was really fun for the first 20 shots, I doubt I would be a long-term user. Not trying to dissuade you from trying it, just letting you know that it may not be a long term keeper.