If you want auto focus, you will need an AFS lens or its equivalent. I would suggest such a lens if you are photographing fish. For corals and other stationary objects a manual lens would be fine.
Nikon makes two superb macro lenses: the 60 mm f 2.8 AFS and the 105 mm f 2.8 AFS. For most aquarium photography, I would go with the 105. It has more "reach" and superb bokeh.
But you mentioned being price sensitive. There are third party manufacturers that make macro lenses that have high quality : tamron, tokina and sigma. These lenses are far cheaper than Nikon glass. You may even look at Adorama or B&H for used lenses. Often even somewhat beaten up lenses perform just as well as new lenses for a considerable discount.