Thanks about the ledge. I won't rescape this tank when I move (or I should say my move and rescaping of this tank don't affect each other). I'll do it whenever I get the time.
The rock ledge is just a progression from what I had already done. I couldn't keep the rocks to stay where I wanted them, so they are actually encased in a plastic mold I made. I used hot glue to hold the rocks in place.
So what I hope I can do is take the whole thing out and glue more rocks on the front side of it. Then rather than burying it in the substrate like a river, I'll use it like a retaining wall to create two depths of substrate which I hope will add to the illusion of scale.
I won't know if it'll work until I try it, but I have nothing to lose other than some time, a few rocks and burnt fingers from the hot glue!