I think Louis has a good point about the perception of depth.
I am surprised this post has been passed by, perhaps it is a language problem, but I find the point is clearly made!
It would not be possible in the current MA with a large central feature, nor indeed required, since the tank depth is only 12 inches. But in a future version, or in the GA, the gravel floor could be used to great advantage to give the illusion of depth! The
'3-D generic question' thread is covering similar ground.
(No pun intended! 
)
At the present time, the impression of depth given by the gravel in GA contradicts the impression given by the fish.
Why not continue the gravel, reducing progressively in size, up to nearer the half way point? - and
behind the rock!
As is said in the
'3-D generic question' thread, ..... it's all about creating an illusion!