This is an example for a buddy that was asking me some camera questions.
First picture with a
nikon nikkor 105mm f2.8 macro lens.
Second picture with a nikon nikkor 105mm f2.8 macro PLUS
nikon TC-20E III teleconverter. The teleconverter essential increases the focal length of the lens, in this case making the 105mm a 210mm lens. Drawbacks are is there is more glass between the image and the sensor, which reduces the available light.
Third option we discussed was using extension tubes. Extension tubes decrease the required distance a lens needs to focus. In the case of a fish tank, extension tubes aren't ideal since a closer focusing distance means you have to actually get closer to the subject. And unless you have a nice underwater housing for the D80 that isn't happening.
In the end the result is usually the same, a magnified image as compared to the lens alone. They just go about different ways of accomplishing it. For your needs, a good macro lens and a teleconverter would be all you need. And honestly, the teleconverter would likely be overkill. Just cropping the image would likely be enough.
Cheers!