looking for a fish eye lens

i have a canon rebel xti. i know, it's 1.6x cropped so it won't like that 'fishy' with the 15mm canon

the only problem i'm having is the aperture, the canon 15mm is constant at 2.8 while the tokina is 3.5-4.5 - making it much more difficult in low light situations, which i encounter a lot

what to do??? i love the quality of the canon 15mm, but i would like the range of the tokina.