Originally Posted by thegundog
I think it is a little heavy on the rock and adding wood would provide more balance.
Some floating plants, maybe forgbit, would also look good.
Taller plants (artificial or real) in the corners would define things a little more.
Lastly, adding some black background to the back of the glass would really help to perpetuate the beautiful colors of your guppies.
A really easy, fast growing stem plant for the corners to check out would be water wisteria. It's cheap, looks nice, doesn't shed all over your tank (like hornwort, the christmas tree of aquarium plants) and is really easy to grow.
Backgrounds can be done super cheap, too, and you'd be amazed what a difference it makes. I just picked up a sheet of black posterboard for 1.00$ that I hung with plain old tape from the upper rim of my tank (a few cuts involved, but a sharp pair of scissors, a straight edge, and a razor to score my line made short work of it).
Something to be mindful of with adding driftwood is that it will most likely leach tannins into your tank. They're not harmful, but they will make your water varying shades of yellowy-brown. You can minimize that by boiling the @)%&@! of out it for 1-3 hours (changing water once-twice) and then soaking it in decholrinated water for a few days (changing the water out every 1-2 days). You'll be able to tell from the water you're dumping when you've hit the amount of tannins you're ok with.
Edit: by a few days I mean up to weeks possibly. The last driftwood arrangement I added was leaching tannins slowly enough for my liking after about 5 days, but the first ones I did took 3 weeks! Longer boil times seem to shorten the soak period, but you're probably also decreasing the lifespan of it a bit in the process. YMMV