- if the tips color is dark green (or black), it's probably algae. more likely is GSA (Green Spot Algae). If this is the thing, it's probably will affect not only the tips, but mostly on the area which exposed to direct lights too much. Just cut the affected leaf and the new shoots will comes out.

- if the tips like been teared or holes or like being bitten, it's probably lack of Pottasium. Just dose a bit more K in the tank. Trim the affected leaf as they'll have a hard time to recover but at least you'll prevent from other leaf to be affected.

-if the leaf going to like transparent or whitish (very light green), erm..this mostly happens to a new leaf and it;s not happening to its tips only. the whole leaf will. if this is the thing, it's normal. the leaf isnt matured yet. they'll turn to green by nature later on what it's time. sometimes it does happens to old leaf but very seldom.

- if the java fern is kinda new in the tank, perhaps they're just trying to acclimate with ur tank conditions. some leaf might get a problem but the plant itself is quite hardy. just cut the affected leaf off. that's why when we putting a fern into our scape, we will cut off the old leaf, leave it with few leaves only and let the new leaf grows out with the tank condition by itself. once the new ones already matured, cut the old leaf (the one that havent been cutted in the first round) and wait for new leaf again. let this cycle continues

- other symptom of java fern where some part of the leaf turn brownish. i never encounter this nor see this before, but I've read somewhere from tom barr forum and he explains that it's due to competition of Co2. this can be seen when your other plants is thriving and only ur java fern affected. this always happens to low-tech tank where no co2/carbon supplied but at the same time, amount of light supplied is lil higher than it should. the way he mentioned to cure this is lowering your light. some people put a blockage from the light to the tank (like tissue or plastic), some just buy a lower light instead.