Africangrey,
that lens is pretty useless for true "macro" shots because it doesn't get anywhere near the 1:1 magnification ratio!
however, it's a great lens for all-around camera work. You can read a very in-depth review on that lens
here.
Additionally, for macro work, try sigma's 105mm f2.8 or tamron's 90mm f2.8
If I was a nikon shooter who wanted to avoid changing lenses as much as possible, didn't want to pony up the cash for pro glass, and wanted to be able to get great aquarium shots but still have a lens to capture "everything" else (and of course, if I had the money to afford it).
My bag would have the Nikkor 18-200mm and the Tamron (or Sigma) macro listed above.
hth,
Matt
ps - you might try selling that 18-135mm for a couple hundred bucks or look for a local photography club in your area and see if anyone wants to buy it there or trade you for it.
Additionally, w/ commander mode on the d80, look into getting an SB-800 or 600 and using the flash remotely above the tank. Stunning results that way from what I've seen.
