I definitely agree with using the macro lens. If it's the same one I have, it's a pretty good lens. The aperature benihana suggested will give you much more depth of field as well. I don't know if you necessarily need to go all the way up to f/22, but unless you're shooting something oriented straight up and down (almost never the case), you'll generally want to stay away from the smaller f-stops. I'd also add that you're going to need a fairly long exposure time, so make sure all of the pumps are off and keep using the tripod (I always hold my breath just for good measure).
Also, make sure you're shooting as straight on as possible and you don't have any backlight on the tank, as from an window, TV or even other lighting in the room. A few of the pictures look a bit out of focus as well. The larger f-stop setting (narrower aperture) will help. You still may have to take it off autofocus. The default setting on most cameras uses an average across various zones in your field of view to focus, which can be an issue especially for macro shots where your subject is often significantly closer to your lens than your background.
The final touch is a little post editing. Adding a bit of shadow and saturation along with tweaking the color temperature can really make the pictures come to life.