[quote author=Paradox link=topic=3125.msg34248#msg34248 date=1203397956]
How do you get it so perfectly black in the background?
[/quote]
first the backgrounds of the tanks are black, so that helps.
by shooting in manual I can adjust the exposure to darken the background.
ideally you want the subject to be as far from the background as possible. I do a few test shots and adjust the shutter speed so the background is dark, then adjust the aperture so the subject is lit well.
in photoshop using curves you can then boost the contrast further, this separates the subject from the background nicely.