I'd definitely go for something like the Sigman 90mm macro (or better yet the Canon 100mm, but that's a bit pricier) instead of the 70-300. A telephoto can often make for a decent substitute for a macro lens in a pinch, but as others have stated, the 70-300 isn't the greatest of lenses, and you really start seeing the difference in image quality when you start talking telephotos. I wouldn't worry as much about aperture, as you're usually stopped down a bit for macro photos anyway, but if you're looking specifically for something to shoot your tank, that definitely would not be my first (or second, or third) choice. I used to own the Canon 70-300, which performed similarly from what I understand, and actually did quite well if you shot at around f/7.1 and avoided going beyond 200mm, but ultimately I just saved my nickles and dimes and bought the Canon 70-200L f/4. Of course, I don't use that lens for aquarium photography either.