i purchased a "vivitar" 100mm macro and get good shots.
it cost about 130 bux online.
i have the 50mm canon macro. it is a much better lense, but not enough reach for aquarium shooting.
the tamron and sigma are both good macro lenses as well.
technique will be in the end as important as any lense you buy.
here are shots with the 100mm vivitar on a D rebel.
www.lakesidestudio.org
look under "aquatics"