Mysterious necrosis of java fern
 

My 55 gallon tank has been set up for about six months now, and the
java fern i added near the beginning was growing very healthily until
over Christmas break, when it suddenly started dieing back. I removed
it to a separate heated bucket to see if it was being eaten and to try
clean water, but the dieback has continued. A month ago it looked
like this:

http://web.mit.edu/lovell/www/aquaria/java.htm

But now it looks even worse. Can anyone diagnose this condition?
I've tried putting macro and micronutrients in, but perhaps i missed
one. Anyway, it was a very beautiful plant one, and i'd love to save
it if possible. I appreciate any help.