CO2! That is the magic word. For that big a tank Excel might be a bit much to pay for, but a two bottle DIY CO2 system could do wonders towards encouraging the plants to grow faster. Still, I would add some cheap stem plants too, like Hygrophilia difformis or Ludwigia Repens. The fish would probably keep them uprooted most of the time, but they still grow that way. First of all though, as sarahboborah said, clean it out, getting rid of as much of the algae as you can, even tossing any plants that are totally covered with it. And, get some fertilizers from Greg Watson, and start a regular dosing regime, plus a weekly water change. Planted tanks without lots of algae are a lot of work, in my opinion.