[aquaphish]

Since you have the flat rocks, and if you want to use them take a hammer to them and break them up into smaller chunks. Once this is done then stack them up like a wall. If you can do say one corner of the tank, about 1/3, with an elevated area using the rocks as a wall this will give you a better look if you want to use the rocks. You will now have an elevated like plateau that you can add plants.