Do you honestly think the lake looks like the bottom shot?
Place coral grit then look to putting two lots of rocks together. One on either side of the tank, depending on how big it is. If a really large tank, 1,5 - 2 metres, then you could look at a third clump in the middle. I changed my tanks from a single row across the tank to this arrangement and the fish seem to enjoy it a whole lot more. They will swim from the one lot to the other.
In the spaces between you can then plant some Vallisneria which they will dig up on a regular occassion. I would also look at some coral pieces after haveing placed the rocks.
The top pic looks great and some good looking fish too.
If my digital was working I would post a shot of the way my tanks are right now. Put in a couple of Synodontis multiplunctatus as they are always really busy darting around the tank. Adds something different to a tank. Please post a pic once you have this tank set up.