Ditto what Coyne said. In all design, whether it is a quilt, painting, room, garden, or aquarium, good design is about a balance between unity and variety. One of the best ways to unify an aquarium is to use rocks of the same color and texture--or multiple objects of similar colors (your driftwood and the little clay-colored shelter are in the same color family, for instance). Add more plants around the back and sides, keeping an eye on the color you already have--the reds and greens. It looks like a great start, so go with the best in design that you already have and build upon it.