The D80 is an EXCELLENT camera. As for which lens to get - it depends on what you want to do with your camera.
For taking photos of your fish, nothing beats a good macro lens (Nikon calls them micro). Nikon has two GREAT micro lenses, the 60mm and the 105mm. Both will take wonderful photos, but the 105mm has VR (vibration reduction) which helps if you intend to shoot hand held (without a tripod) but it is not the best choice for tight areas (like my fish room). The 105mm is also more expensive.
For a walk around, all occasion lens I like my kit lens. It is the 18-135mm and it takes nice photos. I have used it to take photos of my fish, my family and some animals around my house. There are a lot of nice, reasonably priced lenses that will do the job in this category.
If you like the eagle photos Bobby has been posting, you are going to need a long zoom lens. At a minimum I would suggest the 70-200mm (Bobby and I both have this lens). A better choice would be a staright 400mm or 500mm lens. Good zoom lenses start at about $1500 and can cost over $10,000. Lenses are where most of the money goes! Just can't beat a good lens!