I think it would look nice to add the stones on the left side. Adding the stones would balance it out.
If you wanted, you could stand some of the stones vertically, so it looks like a mountain range. The stones remind me of rocky mountains.
You could create small cliffs, add some willow moss hanging over the edge. Lay some smaller rocks on the side and add some green foreground plants in front and some tall grassy plants all along the back. Leave the middle bare, like a valley.
Before you do all this, you could paint the back a dark gray, dark blue or black, depending on what you like. I use inexpensive craft paints, bought at walmart, and a small sponge roller. It's super easy to paint the glass in back and creates a dramatic effect. The craft paint easily comes off when dry (razor blade to start) if you don't like it. I bought my nephew and nieces fish tanks and we painted the backs, black for the boys and blues for the girls. It was a fun project. I tried a moss green, but it made the water look green, so don't recommend that. I think a dark gray or black would contrast nicely with the light colored stones and green plants.