Brown Discolouration on a Plant

I know so little about plants but I recently decided to branch out and
try to put a few in my community tank. I saw a plant that I really
liked and added it to the tank even though the LFS could not tell me
the plant's name. That was two months ago. The plant is a fairly
light green with a long stalk (actually more than one long stalk, as
there are 4 of the plants planted together and a large fern like
structure at the top with many small leaves all facing up. The top of
the plant has started to turn brown. Now the brown discolouration is
so dark that it is almost black. The plant sort of looks like it was
burned on the top. The rest of the plant appears uneffected. I have
4 other types of plants in the tank and none of the others are having
this problem. The plant is under the lights but my lighting is not
very strong and in fact I had worried that it might not be strong
enough to support plants. All of my other plants seem fine though
they are not growing, just maintaining themselves.

Any ideas?