Yes you do. You just nailed it. A macro lens. There are a number available, both Nikon and 3rd party.
Having gone through the exploration phase myself, I can honestly say that no matter how much research and spending you do it comes down to practice and a few fundamentals. You don't need to find a "best macro lens", you just need a macro lens (true macro 1:1) that works at the focal length you would like to shoot at. If you are shooting things close to the glass or are shooting larger corals you will want a shorter focal length. If you want to go 1:1 on frags towards the back of the tank, you are looking at a 150-200mm macro. The 100mm/105mm are a great medium if you are unsure, they are well priced, and they are true macros.
These are the macro lenses I have owned:
4/3 Olympus 35mm macro f/3.5
4/3 Olympus 50mm macro f2 (1:2)
4/3 Olympus Sigma 105 macro f/2.8
Canon MP E 65mm macro f/2.8
Canon 100mm macro usm f/2.8
Canon Sigma 150mm macro f/2.8
Canon Sigma 105mm macro f/2.8
Canon Sigma 180mm macro f/3.5
m4/3 Olympus 60mm macro f/2.8
Nikon 105mm macro f/2.8
Tokina 300mm reflex macro constant f6.3