Have you got any additional lighting for the shots, or would you use the tank lighting only? If just normal lights, on a 20D I'd stick it on ISO 400 or so (don't want it too high because of the grain, and too low will need a long shutter speed). Keep the aperture fairly wide (maybe F8) so you keep reasonable depth of focus, while not needing like a 30 second exposure. Then match the shutter speed to that, looking at the screen/histogram to make sure you're not over or underexposing.
Use a tripod and self timer because if you use the normal tank lighting for the shots, your shutter speed will be fairly low/long and you'll get a blurry photo holding by hand.
Do it (take photos) in a dark room to avoid reflections off the glass. I also wrap up in a black blanket so that when I use flash over the tank I don't get reflected.
Wide angles will enable you to increase perspective and can make the tank seem larger, however too wide and it will distort the image (straight edges will seem curved).
I'm sure other people will have more tips for you