Usually use a macro lens so right to the glass wall. The flash, with diffuser off camera, coming from above. I don't do it enough so I haven't bothered to work out a flash holder that is easy to position.I need to learn more about using flash with my fish pics. Do you put your flash at an angle to the camera?
How far from the tank do you keep the front of the lens?
The black background is a function of small aperture and dark BG behind the subject (hard to do in a tank). But you can darken it in post.