Cichlas;1278824; said:Don't forget that the aperture is likely to be different too. Generally speaking, the larger the focal length the smaller the maximum aperture is. So a long focal length lense cannot let as much light in so can cause problems if there is not enough light about. The smallest number is the important one in that respect, especially for fish work. i.e. f1.8 will be much better than f4.5 for instance.
I think for fish photography a remote flash is more important than having a fast lens. Fast lens are nice to have but you'll have a very thin depth of field if you shoot it wide open and close to the subject. With a remote flash you don't need a fast lens since you'll have plenty of light and you can essentially freeze a fish's movement with higher shutter speed. You'll also be able to use a smaller aperture so that you can get more of the fish in focus.