The trick with shallow tanks is to downplay the elements which give away the dimesions of the tank-- After all you're fighting with a crappy footprint.
The back corners are the biggest offenders. so it's quite important to bring the focus front and center and as FAR away as possible from the back corners. Generally the best way is to plant in a V shape which utilizes the golden section (if you need clarification on that just ask
, and bring the viewers eye to rest at the point in the "V", which will be closer to the front glass than the back.
To further accentuate this focal point, simply allow an opening at the point of the "V" and allow the foreground to "escape" behind it. In other words, the foreground will fill in the space behind the V and dissapear.
To maximize this effect, your hard decor (ie: rocks and wood) must outline the shape of the V.
Another thing that I find helps immensely is to be sure that you keep your rear glass panel as clean and algae free as the you keep the front glass. The black background is an attempt to fool the eye as to where our other "offender's" dimensions lie. In other words, a dark background will make the rear dimension of the tank ambiguous, especially when you bring the "focal point" forward of it.
--if there is any kind of dirt or algae on the back glass, there is no chance of that dimensional ambiguity--
If you know anything about photography, it's a way of increasing the f-stop, there by decreasing the depth of field.
This kind of optical trickery is the only real way to create "depth" a in shallow tank designed for fish viewing.
The "wall of plants" works, but is difficult to make interesting. Luis Navarro did an admirable job for last years AGA contest, though. The other thing to keep in mind for this type of project is that you won't be able to use large plants like swords and Apons. They take up too much depth-- often filling up an entire side of a 4' tank. Also, hard decor is of utmost importance for this type of disply.
Good luck and post us some pics!