First of all, the fifth rock could be placed upright in the back right corner, giving that big, blaring blank spot some interest. A low-growing plant could be contrasted against it. Another possibility I see is flipping the driftwood 180 degrees so that the eye travels in a sort of lean, backwards "S", and place a small stone in the left front corner area (never all the way in the corner) to give the eye a starting point. Of course, the placement of the plants will also affect the composition with size, shape, texture and color variations.
Think of the composition as if you are building a garden path for a visitor to follow, but you will be building this path in three dimensions for the eye to follow in two dimensions. You want the garden path to be interesting at all points, with small points of interest along the way, but you want the visitor to be encouraged to go a little further along the path to see what is next. Also you want the visitor to follow the path subconsciously, without having to think about it, because that is what makes it pleasing--letting the visitor relax and find visually delightful aspects naturally, without having to "work" at deciding where to go next. (Incidentally, this is the basis of what makes us enjoy anything; we are comfortable with something or someone because we don't have to think about what to do next and it just "comes naturally" to us.)
You can lead the visitor to a particular point that provides interest to linger and ponder over (as with a particularly pleasing rock, plant or piece of driftwood) or you can lead them back to the beginning, to retrace the path and focus more intently on particular features if they choose. Remember, the path is along two dimensions although the scene is physically in three dimensions, and the eye will natually go from one point of interest to the next point of interest that is closest VISUALLY, no matter if it is physically further away, as in going from an object in the front of the tank to an object in the very back of the tank first, even though there is an object physically closer to the front, in the middle of the tank. If it would be measurably closer in a photograph, then it is visually closer. Of course, brightness, contrast and color can also be used to lead the eye, as well as form--anything that is interesting to the eye can be used to catch its attention and lead it somewhere. (This is the principle that sign makers and advertisers depend upon, hoping to catch your attention and lead you off the busy path and down some side street you don't normally travel.)
It is fairly basic, but yet it is often difficult to keep in mind all of the tools you can use, and trying to use too many at once can cause them to work against each other and clash, not a harmonious result. It's usually best to start off with a simple design/eye path in mind. Or just throw something together and look at it, being conscious of where your eye is led, what you like and what you don't like about it. Keep in mind that you may like two entirely different things alot, but they may never be able to "get along" with each other in the same space, kind of like your pet cat and your? pet fish, hamster, bird or dog. Let's face it; some things, like cats, can't be relied upon to get along with others no matter how much you like them, so you just have to keep them separated and give them their own space. In the case of your composition, you have some very nice rocks and a very nice piece of driftwood, but each might be better off in their own aquarium, so that they don't compete and distract from each other, like two fish that don't get along.
Sorry if this was a bit philosophical, but that's the nature of art, and planted tank composition is an art form. Hope this helps!
Olskule