Why does my Amazon Sword look so unhealthy?

I've had it for about a month. I have an 18" 15watt fluorescent light in the 10g aquarium. I've tried to find a light with higher wattage, but I haven't been able to find one that would fit. I've cut off the majority of leaves in the middle because they got too high, but I haven't touched the outside leaves. All the leaves are now turning brown around the edges, and they just have a dried out look to them. I add plant food once a week (API Leaf Zone), and this weekend I'm actually buying plant substrate to mix with my current sand substrate.
http://i275.photobucket.com/albums/jj293/irtreefrog/DSCN2023.jpg
It's hard to see in the picture, but you can kinda see the brown on the edges.

Do you think I should just cut all of the leaves off and let it re-grow?
 
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