Like voidraven said, the Nikon 105 macro or Sigma 150 macro are both coveted lenses for macro work. Plus they can be quite useful for portrait work!
Using a tripods, turning pumps off, proper focus and depth of field, shooting in raw so you can adjust white balance afterwards, turning off all the lights in the room while taking pictures, wearing dark clothing while shooting, etc are all tips that can make an awesome picture.
It sounds crazy but if you take a large dark blanket and make a little fort around the tank to block any stray light that may interfer with your photo potential, you can get some great pictures!