Could be the start of the plant starting to propagate. Look for tiny leaflets. If not it might be some sort of deficiency (K?). I'd wager on propagation as java fern is quite hardy. Can you post a better pic.
Yeah, like he said above...java fern tends to be extremely hardy & stable. Is the water temperature stable in your tank? The only time I've seen my java fern that black was once when one of my heaters malfunctioned and overheated my tank.
Ok I don't know what is happening to my Java Fern it is turning black! Please someone tell me what is wrong ? And should I trim all the leaves off or just let it alone?
I have that same condition occur to my various Microsorum pteropus species from time to time. I have been unable to isolate the exact cause, but I am leaning toward a deficiency issue. I find maintaining a good fert program decreases the frequency of occurrence. I am leaning toward nitrogen, potassium, or hardness (magnesium / calcium) as possible causes.
When the condition occurs I trim off the effected leaves as soon as I see the discoloration; sometimes this means removing most of the leaves from a rhizome. As long as the rhizome is healthy new leaves start to sprout in a week or two.