Your tank looks really good, the only thing I like to do, is to add a central focal point. But don't worry about that as it looks really good, and both your fish have designated areas which will save a lot of hassle in the long term.
Ok well I like am Amazonian theme to my tanks. Lots of green, brown and white with Tanic Acid all over lol. All my tanks are setup this way. What I would do to your tank is this: Change to a tan sand bed instead of large river rocks, add some small pieces of driftwood as a here and there thing and have one large peice a little off center because if you have a Focal point in the middle it just looks dumb. Also have some darker colored rocks surrounding the bottom of the large peice of DW and scatter some stones around. BUT since you have it set like that I an goinbg to say add some more slate for caves and lean the largest piece of slate up against the left corner.