[biker]

Because I found that using species of Najas is not enough to create a scene gradually changing in contrast, I use other similar looking plants to support them.

 

[Raul-7]

Nice "contrast", although it doesn't really have that impression...Is that a flower in the bottom right corner?

 

[aquatic-store.com]

Looking good.
I agree the pic has no definite boundaries that stick out.
Maybe background all plants with similar structures and break up the forground with other plants in between