having used both the 100mm and the 60mm canon usm macros for aquarium work, i'd like to note the following observations...
i am a fan of both of these lenses. they are both very good macro lenses with very similar sharpness and overall quality. beyond aquarium work they are also both good walk-about lenses and are both great for portraits. the 60mm is cheaper.
my biggest notes for aquarium work is that the 100mm has significantly more "reach" into a tank. i recommend this lens if you are shooting from the front of the glass into the tank. the 60mm is better for shallow top-down shots, as it allows you to get closer to the coral - an especially important consideration if you have aquarium lighting to dodge. if you are shooting frags in a shallow frag tank, the 60mm is the way to go imo.
also, you can add extension tubes to the 60mm to get it to operate like the 100mm for extra reach in front of tank shots... another consideration.