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Definitely add more plants. Fill that sucker halfway or more full of plants. But first make sure you have adequate lighting.
Also with any stem plants you may get, I'd separate them a little bit so that they grow and fill in the area, plus if you leave them bunched up the bottom may rot off.
If your light levels are marginal, your plants will stay alive, but they won't grow much. Many people get frustrated when they first try live plants because they don't have enough light and the plants either look miserable or die. You'll want 70-80W of fluorescent light over your 55g to get good results with a variety of plants. Even low-light plants do better under more intense lighting.
Looking at your setup, I'm doubting you've got enough light. If those are incandescent fixtures, you're in luck. CFL bulbs screw right in and are a very cost-effective upgrade. If they're single bulb fluorescent fixtures, you can either retrofit them or try a different light fixture. A two-bulb 4ft fluorescent shop fixture is a cheap upgrade, but you'll have to run the tank with an open top or get glass canopies.