You can actually take decent pictures with the 18-55 lens if you follow some basic guidlines. When I first got my D40 it came with the 18-55 lens and I used it several weeks, not a bad lens, just not the best lens for macro images.
1) turn off the pumps in the tank, you don't want any water movement or you'll get image blur or visual noise.
2) clean the glass on the INSIDE of the tank, very critical as there is always a thin film of algae on the inside of the tank and this will cause blurr.
3) USE A TRIPOD > can't even consider not using one
4) use the timer on the camera, the simple action of pushing the shutter button will cause the camera to vibrate and this causes image noise. Sounds trivial, but it's more critical than most people think
5)USE MANUAL focus, autofocus never seems to capture the correct details.
A telephoto lens could be used, but you'll have to stand so far back from the tank that's a waste of money. Trust me, I went that route as well.
Try using the camera in "A" mode, not macro. If you need more advice on the settings, just ask away.