I like the in-and-out, the open channels between the rock. Might pull something out about a third of the way in on the right? and let the sand flow all the way to the back, creating two reef formations. Might also try turning the right-hand formation around, so that the branches angle into the center, rather into the glass. (If that's a viewable panel, then forget the last sentence completely!)
Overall, I think you've got a nice sweep and movement here.
~Bruce