I would try to add a little more visual depth. At least from the picture it looks pretty flat, like a rock wall. It also looks to have one fairly uniform height. I'd use the rule of thirds to vary the heights and draw the eye from peak to peak directionally across the tank.
I'm not sure how big this aquascape is compared to the tank but I'd suggest you play with the negative space a bit. It's easy to fall into the trap of adding more rock in an attempt to make an beautiful aquascape, when in fact less is often more. having some more open space in an aquarium can add a lot of impact and drama to intricate rock works, caves and overhangs.
There are a few excellent aquascaping posts out there, but I'd suggest checking out some photography blogs. They often talk about how to compose a shot for really dynamic photos. If you imagine each(viewable) side of your tank as a still "landscape" shot, these photography lessons can be really applicable.
