It depends. I use a Canon Rebel XT, so I can't give specific Nikon recommendations, but currently I use the kit lens that came with the camera; 18-55mm f/3.5-5.6. I like it for it's ability to take a wide shot of a full tank at around 18mm without having to stand back far, and it having a medium range for average fish and coral shots. It has worked well for me so far, but in the near future I'll be looking to also get something like a 100mm macro lens to take some nice close ups.
Hopefully that helps a bit.